Solid State Bidirectional Light Sheet for General Illumination

ABSTRACT

A solid state light sheet and method of fabricating the sheet are disclosed. In one embodiment, bare LED chips have top and bottom electrodes, where the bottom electrode is a large reflective electrode. The bottom electrodes of an array of LEDs (e.g., 500 LEDs) are bonded to an array of electrodes formed on a flexible bottom substrate. Conductive traces are formed on the bottom substrate connected to the electrodes. A transparent top substrate having conductors is then laminated over the bottom substrate. Various ways to connect the LEDs in series are described along with many embodiments. The light sheets may be formed to emit light from opposite surfaces of the light sheet, enabling it to be used in a hanging fixture to illuminate the ceiling as well as the floor. The light sheet provides a practical substitute for a standard 2×4 foot fluorescent ceiling fixture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. application Ser. No. 12/870,760,filed on Aug. 27, 2010, entitled Solid State Light Sheet for GeneralIllumination, by Louis Lerman et al.

FIELD OF INVENTION

This invention relates to solid state illumination and, in particular,to a light sheet containing light emitting dies, such as light emittingdiodes (LEDs), that may be used for general illumination.

BACKGROUND

High power LEDs are the conventional choice for general solid statelighting applications. Such high power white LEDs are extremely brightand can have luminous efficacies between 100 and 200 lumens/watt. Theinput power of a single high-power LED is typically greater than 0.5watt and may be greater than 10 watts. Such LEDs generate considerableheat since they are only about 1 mm² in area, so the required packagingis fairly complex and expensive. Although a bare high-power LED chiptypically costs well under $1.00 (e.g., $0.10), the packaged LEDtypically costs around $1.50-$3.00. This makes a high output (e.g.,3000+ lumens) solid state luminaire relatively expensive and not acommercially feasible alternative for a standard 2×4 foot fluorescentlight fixture, commonly used in offices. Further, the optics required toconvert the high brightness point sources into a substantiallyhomogeneous, broad angle emission for an office environment (where glarecontrol is important) is extremely challenging.

To greatly reduce the cost of a large area, high lumen output lightsource, it is known to sandwich an array of bare LED dice between abottom sheet having conductors and a top transparent sheet havingconductors. The LEDs have top and bottom electrodes that contact a setof conductors. When the conductors are energized, the LEDs emit light.The light sheet may be flexible.

The Japanese published application S61-198690 by Hiroshi (filed in 1985and published on 3 Sep. 1986) describes a light sheet using a plastictransparent front substrate having thin wires formed on it. A bottomsubstrate also has thin wires formed on it. An array of bare LED chipswith top and bottom electrodes is arranged on the bottom substrate, andthe front substrate is adhesively secured over the LED chips. LED chipsat the intersections of energized perpendicular wires emit light.

The Japanese published application H08-18105 by Hirohisa (filed in 1994and published on 19 Jan. 1996) describes a light sheet using atransparent front substrate having transparent electrodes (ITO)connected to metal strips. A backside substrate has metal conductorsarranged in strips. Bottom electrodes of bare LED chips are bonded tothe metal conductors on the backside substrate, such as using solderpaste and reflow. A stamped “epoxy hotmelt adhesive” is provided on thebackside substrate surrounding the LED chips. A liquid epoxy moldingresin then fills in the inner area within the epoxy hotmelt adhesive.The hotmelt adhesive is then softened, and the front substrate is thenaffixed over the LED chips using the hotmelt adhesive and the curedmolding resin. Applying current to the perpendicular strips of metalconductors on the opposing substrates energizes an LED chip at theintersection of two conductors. In one embodiment, the front andbackside conductors/electrodes are formed over the entire surface, soall the LED chips will be energized simultaneously for use as anilluminator.

U.S. Pat. No. 6,087,680 to Gramann (priority filing date 31 Jan. 1997,issued 11 Jul. 2000) describes a light sheet using “elastic plastic” topand bottom substrates. Thin metal conductor strips and electrodes aresputtered onto the substrates or deposited in other conventional ways.Bare LED chips are provided with top and bottom electrodes. A conductiveadhesive is used to adhere the bottom electrodes of the LED chips to thebottom substrate electrodes. A “coupling medium” fills in the spacesbetween the LED chips and is used for increasing light extraction. Thecoupling medium may be a liquid adhesive such as epoxy, resin, orsilicone. The top substrate is then affixed over the LED chips, wherethe adhesive coupling medium affixes the substrates together andencapsulates the LED chips. Gramann describes the top and bottomsubstrates being “a structured conducting foil being formed essentiallyof plastic” that is capable of “plastic or elastic deformation,” so thelight sheet is flexible.

Various patents to Daniels et al. have been issued relating to theearlier light sheets described above. These include U.S. Pat. Nos.7,217,956; 7,052,924; 7,259,030; 7,427,782; and 7,476,557. Daniels'basic process for forming a flexible light sheet is as follows. Bare LEDchips having top and bottom electrodes are provided. A bottom substratesheet is provided with metal conductor strips and electrodes. A hotmeltadhesive sheet is formed separately, and the LED chips are embedded intothe adhesive sheet. A transparent top substrate sheet is provided withmetal conductor strips leading to transparent ITO electrodes. Theadhesive sheet, containing the LEDs, is sandwiched between the top andbottom substrates, and the three layers are laminated together usingheat and pressure so that there is electrical contact between the LEDchips' electrodes and the opposing substrate electrodes. The process isperformed as a continuous roll-to-roll process. The roll is later cutfor a particular application. The LED chips may be arranged in a patternto create a sign, or the LED chips may be arranged in an array toprovide illumination.

In an alternative Daniels process, described in U.S. Pat. No. 7,259,030,a bottom substrate has an adhesive conductive sheet over it, on which islaminated a double sided adhesive sheet with holes. The LEDs are thenplaced in the holes, and another conductive sheet is laminated over thedouble sided adhesive sheet. The top transparent substrate is thenlaminated over the conductive sheet. The LEDs are electrically bonded tothe two conductive layers by a high pressure roller at the end of thelamination process so the LEDs are connected in parallel.

Problems with the above-described prior art include: 1) little or noconsideration for removing heat from the LEDs; 2) excessive downwardpressure on the LEDs during lamination; 3) total internal reflections(TIR) caused by differences in indices of refraction; 4) difficulty inproviding phosphor over/around the LEDs to create white light; 5) noconsideration for enabling the light sheet to be optically functionaland aesthetically pleasing if one or more LEDs fail (e.g., shorts out);6) unattractive non-uniformity of light and color over the light sheetarea; 7) difficulty of manufacture; 8) unreliability of LED electrodebonding; 9) excessively high lamination pressures needed to create widelight sheets; 10) inefficiency due to light absorption; 11) difficultyin creating series strings of LEDs; 12) impractical electrical driverequirements for the LEDs; and 13) inability of the light sheet to emitlight in other than a Lambertian pattern. There are other drawbacks withthe above-described light sheets.

What is needed is a cost-effective light sheet that can substitute for astandard fluorescent lamp fixture or that can be used for other lightingapplications.

SUMMARY

Light sheets and techniques for fabricating the light sheets aredescribed that overcome drawbacks with the prior art.

In one embodiment, a flexible circuit is formed as a strip, such as 3-4inches by 4 feet, or in a single large sheet, such as a 2×4 foot sheet.On the bottom of the sheet is formed a conductor pattern using platedcopper traces leading to connectors for one or more power supplies. Atcertain areas of the flex circuit, where bare LED chips are to bemounted, metal vias extend through the flex circuit to form an electrodepattern on the top surface of the flex circuit. In one embodiment, thepattern is a pseudo-random pattern, so if any LED fails (typicallyshorts) or any electrode bond fails, the dark LED will not benoticeable. In another embodiment, the pattern is an ordered pattern. Ifthe light sheet spreads the LED light laterally, a dark LED may not benoticeable due to the light mixing in the light sheet. The metal viasprovide heat sinks for the LEDs, since the rising heat from the LEDswill be removed by the air above the light sheet when the light sheet ismounted in a ceiling. The metal vias can be any size or thickness,depending on the heat needed to be extracted.

In another embodiment, the sheet comprises a highly reflective layer,such as an aluminum layer, having a dielectric coating on both surfaces.The reflective sheet is patterned to have conductors and electrodesformed on it. The aluminum layer also serves to spread the LED heatlaterally. The dielectric coatings may have a relatively high thermalconductivity, and since the sheet is very thin (e.g., 1-4 mils, or lessthan 100 microns), there is good vertical thermal conduction. Suchreflective films will reflect the LED light towards the light outputsurface of the light sheet.

Bare LED chips (also referred to as dice) are provided, having top andbottom electrodes. The bottom electrodes are bonded to the metal viasextending through the top of the flex circuit. A conductive adhesive maybe used, or the LEDs may be bonded by ultrasonic bonding, solder reflow,or other bonding technique. In one embodiment, low power (e.g., 60-70milliwatts) blue or ultraviolet LEDs are used. Using low power LEDs isadvantageous because: 1) hundreds of LEDs may be used in the light sheetto spread the light; 2) low power LEDs are far less expensive than highpower LEDs; 3) there will be little heat generated by each LED; 4) afailure of a few LEDs will not be noticeable; 5) the localized LED lightand slightly varying colors will blend into a substantially homogenouslight source a few feet from the light sheet without complex optics; 6)the blue light can be converted to white light using conventionalphosphors; 7) higher voltages can be used to power many series-connectedLEDs in long strips to reduce power loss through the conductors; andother reasons.

Over the top of the flex circuit is affixed a thin transparent sheet (anintermediate sheet), such as a PMMA sheet or other suitable material,that has holes formed around each LED. The intermediate sheet is formedwith reflectors such as prisms on its bottom surface or with reflectorswithin the sheet, such as birefringent structures, to reflect lightupward. The thickness of the intermediate sheet limits any downwardpressure on the LEDs during the lamination process. The top electrodesof the LEDs may protrude slightly through the holes in the intermediatesheet or may be substantially flush. The intermediate sheet may besecured to the flex circuit with a thin layer of silicone or otheradhesive or bonding technique.

The intermediate sheet may also be provided with a thin reflectivelayer, such as aluminum, on its bottom surface for reflecting light.Since the flex circuit conductors are on the bottom of the flex circuit,and the metal vias are only in the holes of the intermediate sheet,there is no shorting of the conductors by the metal reflective surfaceof the intermediate sheet.

In one embodiment, the LEDs have a thickness between about 85-250microns, and the intermediate sheet surrounding the LEDs is about thesame thickness as the LEDs.

In another embodiment, the intermediate sheet is a dielectric sheethaving cups molded into it at the positions of the LEDs. The cups have ahole in the bottom for the LEDs to pass through. The surface of thesheet is coated with a reflective layer, such as aluminum, which iscoated with a clear dielectric layer. The reflective cups are formed tocreate any light emission pattern from a single LED. In such anembodiment, the LED light will not mix within the intermediate sheet butwill be directly reflected out.

The space between the LEDs and the hole (or cup) walls in theintermediate sheet are then filled with a mixture of silicone andphosphor to create white light. The silicone encapsulates the LEDs andremoves any air gaps. The silicone is a high index of refractionsilicone so that there will be good optical coupling from the GaN LED (ahigh index material), to the silicone/phosphor, and to the intermediatesheet. The area around each LED in the light sheet will be the same,even though the alignment is not perfect. The LEDs may be on the orderof 0.25 mm²-1 mm², and the intermediate sheet holes may have diametersaround 3 mm or more, depending on the required amount of phosphorneeded. Even if an LED is not centered with respect to the hole, theincreased blue light from one side will be offset by the increasedred-green light components (or yellow light component) from the otherside. The light from each LED and from nearby LEDs will mix in theintermediate sheet and further mix after exiting the light sheet to formsubstantially homogenous white light.

In one embodiment, the LEDs have a top surface area on the order of100×100 microns to 300-300 microns, and a thickness of 85-250 microns.Therefore, there is a significant side emission component.

A transparent flex circuit is then laminated over the intermediatesheet, where the top flex circuit has a conductor and electrode pattern.The electrodes may have a conductive adhesive for bonding to the topelectrodes of the LEDs. A silicone layer may be provided on the flexcircuit or on the intermediate sheet for affixing the sheets together.The transparent flex circuit is then laminated under heat and pressureto create good electrical contact between the LED electrodes and the topcircuitry. The intermediate sheet prevents the downward pressure duringlamination from excessively pressing down on the LEDs. The intermediatesheet also ensures the light sheet will have a uniform thickness so asto avoid optical distortions.

To avoid a bright blue spot over each LED, when viewed up close, the topflex circuit electrode may be a relatively large diffusing reflector(e.g., silver) that reflects the blue light into the surroundingphosphor. Such a large reflector also reduces the alignment tolerancefor the sheets.

Even if a reflector over each LED is not used, and since the LEDs aresmall and not very bright individually, the blue light from the topsurface of the LEDs may be directly output and mixed with the red/greenor yellow light generated by the phosphor surrounding the LED to createwhite light a short distance from the light sheet.

Alternatively, phosphor may be formed as a dot on the top surface of thetop flex circuit above each LED. This would avoid a blue spot over eachLED. The phosphor/silicone in the holes, encapsulating the LEDs, wouldthen be used just for converting the side light from the LEDs. If lightfrom the top surface of each LED is to exit the top flex circuit forconversion by the remote phosphor, the flex circuit electrode may betransparent, such as a layer of ITO. In an alternative embodiment, thereis no phosphor deposited in the holes in the intermediate sheet, and allconversion is done by a remote phosphor layer on the top surface of thetop flex circuit.

In one embodiment, the LED chips are flip chips, and all electrodes andconductors are formed on the bottom substrate. This simplifies theseries connections of the LEDs and improves electrode bond reliability.

For easing the formation of series connections with LED chips having topand bottom electrodes, the LED chips may be alternately mounted upsidedown on the bottom substrate so that the cathode of an LED chip can beconnected in series to the anode of an adjacent LED chip using theconductor pattern on the bottom substrate. The top substrate also has aconductor pattern for connecting the LEDs in series. Combinations ofseries and parallel groups can be created to optimize the power supplyrequirements.

In another embodiment, the intermediate sheet has electrodes formed onopposing walls of its square holes. The LED chips, with top and bottomelectrodes, are then inserted vertically in the holes so that the LEDelectrodes contact the opposing electrodes formed on the walls of theholes. The electrodes formed in the holes extend to a top surface, abottom surface, or both surfaces of the intermediate sheet for beinginterconnected by a conductor pattern on the top substrate or bottomsubstrate. In an alternate embodiment, the conductor pattern for anyseries connection or series/parallel connection is formed directly on asurface or both surfaces of the intermediate sheet.

In another embodiment, there is no intermediate sheet and conductors arepatterned on top and bottom substrates. One or both of the substrateshas a cavity or groove to accommodate the thickness of the LEDs. Thevertical LEDs are then sandwiched between the two substrates. If theLEDs are thin enough, no cavities are needed to accommodate thethickness of the LEDs since the assembly process can simply rely uponthe plastic deformation of materials to encase the LEDs. The conductorpatterns on the opposing substrates are such that the sandwichingconnects the conductors to couple adjacent LEDs in series. Thesubstrates may be formed as flat strips or sheets, or rounded, or acombination of flat and rounded. In one embodiment, the sandwichedstructure forms a flexible cylinder or half cylinder that contains asingle string of series connected LEDs. The flexible strings may beconnected in series with other strings or connected in parallel withother strings, depending on the desired power supply.

If the light sheet is formed in strips, each strip may use its own powersupply and be modular. By fabricating the light sheet in strips, thereis less lamination pressure needed, and the lamination pressure will bemore uniform across the width of the strip. The strips can be arrangednext to each other to create any size light sheet, such as a 2×4 footlight sheet or even a 6 inch by 4 foot or longer light sheet tosubstitute for light sources within a standard fluorescent fixture in anoffice environment. It is common for fluorescent fixtures within a givenceiling cut-out to contain two, three, four or more linear fluorescentlamps. Each light sheet strip may substitute for a single fluorescentlamps and have a similar length. This embodiment of the light sheet cangenerate the roughly 3000 lumens needed to replace the typicalfluorescent lamp and, by inserting the required number of strips in avariety of spatial configurations, it is possible to manufacture thelighting fixture with the same flexibility of lumen output to suit thelighting application. The particular design of the light sheet enablesthe light sheet to be a modular cost-effective solution.

Alternatively, it is known that standard ceiling grid configurations forfluorescent fixtures come in discrete sizes such as 6 inches×4 feet, 1×4feet, 2×4 feet and 2×2 feet. It is possible to consider the use ofnarrow 2 foot strips of 1500 lumens each as a standard modular size thatcould potentially be used as building blocks within each of theseconfigurations. Thus, the manufacturer of the final fixture could stocka single size component by which they could conceivably create any typeof lamp configuration and geometry as seen in the majority ofapplications.

Various light strips in a fixture may be tilted at different angles todirect a peak intensity of the light from an associated light strip atany angle. This greatly expands the ability of a composite fixture toshape and modulate the distribution of light in the far-field away fromthe light fixture itself.

Alternatively, a single 2×4 foot light sheet (or sheet of any size) maybe employed that is, in itself, the fixture without any enclosure.

For the case where the lighting fixture offers significant surface area,such as in a 2×4 foot fluorescent light fixture, there is significantroom to blend many smaller LED sources such that their local thermalconditions are better managed than in a retrofit bulb or spot light typelight source where the heat becomes highly localized and thus harder tomanage.

The light sheets are easily controlled to be automatically dimmed whenthere is ambient sunlight so that the overall energy consumption isgreatly reduced. Since individual light sheets may have combinations ofseries and parallel strings, it is also possible to create sub-lightsheet local dimming. Other energy saving techniques are also discussedherein.

The LEDs used in the light sheet may be conventional LEDs or may be anytype of semiconductor light emitting device such as laser diodes, etc.Work is being done on developing solid state devices where the chips arenot diodes, and the present invention includes such devices as well.

The flexible light sheets may be arranged flat in a support frame, orthe light sheets may be bent in an arc for more directed light. Variousshapes of the light sheets may be used for different applications. Thetop flex circuit sheet or the intermediate sheet may have opticalfeatures molded into it for collimating the light, spreading the light,mixing the light, or providing any other optical function.

For some applications, such as for using the light sheet in a reflectivetroffer or hanging from the ceiling, the light sheet is madebidirectional.

In one embodiment of a bidirectional light sheet, the upward emission isUV to disinfect the air, such as from a vent or entering an air returnduct. The bottom emission will typically be substantially white light.

In another embodiment, the LEDs are mounted on a snap-in substrate thatsnaps into a groove or cavity formed in the top substrate. Electricalconnections are automatically made by the snap-in fit.

The light strips may be located in a standard fluorescent tube formfactor for supporting and powering the LEDs using a standard fluorescentlamp fixture. In one embodiment, the tube form factor has a flat top onwhich the light strip is mounted. The flat top is directly contacted byambient air to cool the light strip, or there may be an intermediatelayer between the flat top and the air. The variable emission patternsof various light strips in the tube enable the tube to have any emissionpattern.

Various techniques of removing heat from the LEDs are also described.

Novel methods of encapsulating the LED dies are also disclosed. In oneembodiment, holes are formed in the top substrate aligned with the spacearound each LED die. After the top substrate is affixed over the LEDdies, an encapsulant is injected into the space via the holes in the topsubstrate. Some holes allow air to escape from the space as the space isfilled by the encapsulant.

Other variations are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The below described drawings are presented to illustrate some possibleexamples of the invention.

FIG. 1 is a simplified perspective view of a portion of the light outputside of a light sheet in accordance with one embodiment of theinvention.

FIG. 2 is a simplified perspective view of a portion of the underside ofa light sheet in accordance with one embodiment of the invention.

FIGS. 3-5, 7, 8, 10-14, and 16-19 are cross-sectional views along line3-3 in FIG. 1 showing the light sheet at various stages of fabricationand various embodiments.

FIG. 3A illustrates the flexible bottom substrate having conductors andelectrodes, where the electrodes are heat conducting vias through thesubstrate.

FIG. 3B illustrates a reflective bottom substrate having conductors andelectrodes, where the reflector may be an aluminum layer.

FIG. 3C illustrates a reflective bottom substrate having conductors andelectrodes, where the reflector is a dielectric and where the electrodesare heat conducting vias through the substrate.

FIG. 4 illustrates a conductive adhesive dispensed over the substrateelectrodes.

FIG. 5 illustrates bare LED chips, emitting blue light, affixed to thesubstrate electrodes.

FIG. 6 is a perspective view of a transparent intermediate sheet havingholes for the LEDs. The sheet may optionally have a reflective bottomsurface.

FIG. 7 illustrates the intermediate sheet affixed over the bottomsubstrate.

FIG. 8A illustrates the holes surrounding the LEDs filled with asilicone/phosphor mixture to encapsulate the LEDs.

FIG. 8B illustrates the holes surrounding the LEDs filled with asilicone/phosphor mixture, where the holes are tapered to reflect lighttoward the light output surface of the light sheet.

FIG. 8C illustrates the intermediate sheet molded to have cupssurrounding each LED, where a reflective layer is formed on the cups toreflect light toward the light output surface of the light sheet.

FIG. 8D illustrates the intermediate sheet being formed of phosphor orhaving phosphor powder infused in the intermediate sheet.

FIG. 8E illustrates that the LED chips may be pre-coated with phosphoron any sides of the chips.

FIG. 9 is a perspective view of a top transparent substrate having aconductor pattern and electrode pattern. The electrodes may bereflective or transparent.

FIG. 10 illustrates a conductive adhesive dispensed over the LEDs' topelectrodes.

FIG. 11 illustrates the top substrate laminated over the LEDs, whereside light is reflected through the light output surface of the lightsheet by prisms molded into the intermediate sheet.

FIG. 12A illustrates the top substrate laminated over the LEDs, whereside light is converted to a combination of red and green light, oryellow light, or white light and reflected through the light outputsurface of the light sheet, while the blue light from the LEDs isdirectly transmitted through the transparent electrodes on the toptransparent substrate for mixing with the converted light.

FIG. 12B illustrates the top substrate laminated over the LEDs, where areflector overlies the LED so that all light is converted to white lightby the phosphor and reflected through the light output surface of thelight sheet.

FIG. 12C illustrates the top substrate laminated over the LEDs, whereside light is converted to white light by the phosphor surrounding theLEDs, and the top light is converted to white light by a remote phosphorlayer over the LEDs.

FIG. 12D illustrates the top substrate laminated over the LEDs, wherethe LEDs are positioned in a reflective cup, and where side light andtop light are converted to white light by a large phosphor layer overthe LEDs.

FIG. 13 illustrates the use of a flip chip LED in the light sheet, wherethe flip chip may be used in any of the embodiments described herein.

FIG. 14 illustrates the reverse mounting of alternate LEDs on the bottomsubstrate to achieve a series connection between LEDs.

FIG. 15 illustrates the intermediate sheet having electrodes formed onopposing walls of its holes for contacting the top and bottom electrodesof the LEDs.

FIG. 16 illustrates the LEDs inserted into the holes of the intermediatesheet and the electrodes on the intermediate sheet being interconnectedtogether by a conductor pattern on any of the layers for connecting theLEDs in any combination of serial and parallel.

FIG. 17 illustrates two light rays being reflected off the reflectiveelectrodes on the intermediate sheet or the bottom reflective electrodeof the LED and being converted to white light by a phosphor layer.

FIG. 18 illustrates an alternative embodiment where the conductors forinterconnecting the LEDs are formed on opposite surfaces of theintermediate sheet or on surfaces of the top and bottom substrates.

FIGS. 19A and 19B illustrate the LEDs being connected in series by ametal via bonded to a bottom electrode and extending through theintermediate layer.

FIGS. 20-31 illustrate another set of embodiments where no intermediatesheet is used.

FIGS. 20A and 20B are cross-sectional views of a light sheet or strip,where a channel or cavity is formed in the bottom substrate, and where aseries connection is made by conductors on two opposing substrates.

FIG. 20C is a transparent top down view of the structure of FIG. 20Bshowing the overlapping of anode and cathode conductors.

FIG. 20D illustrates multiple series strings of LEDs being connected inthe light sheet or strip of FIG. 20B.

FIG. 21A is a cross-section of structure that contains a series stringof LEDs sandwiched between two substrates.

FIG. 21B is a top down view of the structure of FIG. 21A showing theoverlapping of anode and cathode conductors.

FIG. 21C illustrates the sandwiched LED of FIG. 21A.

FIG. 22 is a cross-sectional view of a substrate structure having ahemispherical top substrate, where the structure contains a seriesstring of LEDs sandwiched between two substrates.

FIGS. 23A and 23B are cross-sectional views of a substrate structurewhere a channel or cavity is formed in the top substrate, where thestructure contains a series string of LEDs sandwiched between twosubstrates. FIG. 23B also shows the use of an external phosphor layer onthe top substrate outer surface.

FIG. 24 is a schematic view of a series string of LEDs that may be inthe substrate structures of FIGS. 20-23.

FIG. 25 is a top down view of a single substrate structure or a supportbase supporting multiple substrate structures.

FIG. 26A is a cross-sectional view of two substrates connected togetherby a narrow region so the substrates can sandwich a string of LEDs.

FIG. 26B is a perspective view of the substrates of FIG. 26A.

FIG. 26C illustrates the structure of FIG. 26A being supported in areflective groove in a support base.

FIG. 27 is a cross-sectional view of an LED that emits light fromopposing sides of the chip, where the structure contains a series stringof LEDs sandwiched between two substrates.

FIG. 28 illustrates a phosphor technique where the phosphor over the topof the LED chips is provided on the top substrate. FIG. 28 alsoillustrates an optical sheet over the top substrate that creates anydesired emission pattern.

FIG. 29 illustrates a top substrate that is formed to have ahemispherical remote phosphor and reflecting grooves for reflecting sidelight toward a light output surface.

FIG. 30A illustrates an end of a sheet or strip where the bottomsubstrate is extended to provide connection terminals leading to theanode and cathode conductors on the top and bottom substrates forconnection to a power supply or to another string of LEDs.

FIG. 30B is a top down view of FIG. 30A illustrating an example of theconnection terminals at one end of a sheet or strip.

FIG. 31 is a side view of a portion of a longer strip of LEDs showinganode and cathode connection terminals at the ends of two serial stringsof LEDs within the strip so the strings can be either connected togetherin series or parallel, or connected to other strings in other strips, orconnected to a power supply.

FIG. 32 is a perspective view of a frame for supporting a flexible lightsheet strip or sheet to selectively direct light.

FIG. 33 illustrates LED dies that are oppositely mounted in a lightsheet to create a bidirectional emission pattern.

FIG. 34 illustrates two light sheets back-to-back, which may use acommon middle substrate, to create a bidirectional emission pattern.

FIG. 35 illustrates another embodiment of two light sheets back-to-backto create a bidirectional emission pattern.

FIG. 36 illustrates a bidirectional light sheet hanging from a ceiling.

FIG. 37A is a cross-sectional view of a snap-in LED die substrate, whichmay be an LED strip or a single LED module.

FIG. 37B illustrates the series connections formed on the top substratefor connecting the LED dies in series.

FIG. 38 illustrates how a plurality of top substrates may be snappedover a mating bottom substrate.

FIG. 39 illustrates that the bottom substrate may include one or morecurved reflectors along the length of the LED strip to reflect sidelight toward an object to be illuminated. This figure also shows thatthe shape of the top substrate may be domed or an extended domestructure over the bottom substrate.

FIG. 40 is similar to FIG. 37A except that the LED die substrate isfixed in place by a conductive adhesive or solder reflow.

FIG. 41 illustrates a small portion of a bidirectional light sheetpositioned in front of an air vent, where the top emission is UV fordisinfecting air, and the bottom emission is substantially white lightfor illumination.

FIG. 42 is similar to FIG. 41 but the air is allowed to flow through thelight sheet. The light sheet may be installed as a ceiling panel.

FIG. 43 illustrates how optics may be formed in the top substrate on thesurface opposing the LEDs.

FIG. 44 illustrates that red, green, and blue LEDs, or red, green, blueand white LEDs or combinations thereof, may make up the light sheet andbe controllable to achieve any white point.

FIG. 45 illustrates that blue and infrared LEDs may make up the lightsheet, where the blue LEDs are used for generating white light and theinfrared LEDs are only energized while the blue LEDs are off, such as inresponse to a motion sensor, for providing low energy lighting forsurveillance cameras.

FIG. 46A illustrates a laser ablating openings in top and bottomsubstrates for exposing the electrodes of LEDs.

FIG. 46B illustrates the openings of FIG. 46A being filled with metal,or metal filled epoxy, or printing material that is cured to provideelectrical contact to the LEDs and to provide heat sinking.

FIG. 47A illustrates LEDs being mounted with their small electrodesaligned to substrate electrodes to make use of the high positionalaccuracy of automatic pick and place machines.

FIG. 47B illustrates the LEDs of FIG. 47A being sandwiched between twosubstrates without any cavity or intermediate layer due to the thinnessof the LEDs. A series connection between LEDs is automatically made bythe conductors formed on the substrates.

FIG. 47C is a bottom up view of FIG. 47B illustrating the seriesconnections between LEDs.

FIG. 48 is a perspective view of a lighting structure, illustrating howthe LED strips of any embodiment may be positioned in a transparent ordiffusing tube so as to be used in standard fluorescent lamp fixtures.

FIG. 49 illustrates how the tube form factor may be changed to have aflat surface, or any other non-cylindrical feature, for supporting theLED strip and improving heat transfer to the ambient air.

FIG. 50 is a cross-sectional view of a fixture incorporating the lightstructure of FIG. 41, with a light strip being supported by the top flatsurface of the tube and heat escaping through holes in the flat surfaceand holes in the LED strip.

FIG. 51 is a side view of an embodiment where the tube shape is formedby the flexible light sheet itself.

FIG. 52 is a perspective view illustrating that a bidirectional lightsheet may be bent to have a rounded shape to form a partial tube or amuch larger luminaire.

FIG. 53 is a perspective view illustrating a light sheet having a topemission directed toward a top panel, where the top panel may bediffusively reflective or have a phosphor coating.

FIG. 54A is a top down view of a top substrate with holes for fillingspaces around the LED dies with an encapsulant and holes for allow airto escape the spaces.

FIG. 54B is a cross-sectional view of a light sheet showing a liquidencapsulant being injected into the space around each LED die throughthe holes in the top substrate.

FIG. 55A is a cross-sectional view showing a blob of softenedencapsulant material deposited over the LED dies.

FIG. 55B illustrates the softened encapsulant material being pressed andspread out within the space around the LED dies, with any excessmaterial overflowing into a reservoir.

Any of the various substrates and intermediate layers may be mixed andmatched in other embodiments

Elements that are the same or similar are labeled with the samenumerals.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of a portion of the light output side of alight sheet 10, showing a simplified pseudo-random pattern of LED areas12. The LED areas 12 may instead be in an ordered pattern. There may be500 or more low power LEDs in a full size 2×4 foot light sheet togenerate the approximately 3700 lumens (per the DOE CALiPER benchmarktest) needed to replace a standard fluorescent fixture typically foundin offices.

The pseudo-random pattern may repeat around the light sheet 10 (only theportion within the dashed outline is shown). A pseudo-random pattern ispreferred over an ordered pattern since, if one or more LEDs fail orhave a poor electrical connection, its absence will be significantlyharder to notice. The eye is drawn to defects in an ordered patternswhere spacings are consistent. By varying the spacing in a pseudo-randompattern such that overall light uniformity is achieved and where theremay be a low amplitude variation in luminance across the surface of thefixture, then the loss of any one LED would not be perceived as a breakin the pattern but blend in as a small drop in local uniformity. Typicalviewers are relatively insensitive to local low gradientnon-uniformities of up to 20% for displays. In overhead lightingapplications, the tolerable levels are even higher given that viewersare not prone to staring at fixtures, and the normal angle of view ispredominantly at high angles from the normal, where non-uniformitieswill be significantly less noticeable.

An ordered pattern may be appropriate for applications where there is asubstantial mixing space between the light sheet and the final tertiaryoptical system which would obscure the pattern and homogenize the outputadequately. Where this would not be the case and there is a desire tohave a thinner profile fixture, then the pseudo random pattern should beemployed. Both are easily enabled by the overall architecture.

Alternatively, a variably ordered pattern of LED areas 12 may modulateacross the light sheet 10.

The light sheet 10 is generally formed of three main layers: a bottomsubstrate 14 having an electrode and conductor pattern; an intermediatesheet 16 acting as a spacer and reflector; and a transparent topsubstrate 18 having an electrode and conductor pattern. The LED chipsare electrically connected between electrodes on the bottom substrate 14and electrodes on the top substrate 18. The light sheet 10 is very thin,such as a few millimeters, and is flexible.

FIG. 2 is a perspective view of a portion of the underside of the lightsheet 10 showing the electrode and conductor pattern on the bottomsubstrate 14, where, in the example, the LED chips in the LED areas 12are connected as two groups of parallel LEDs that are connected inseries by conductors not shown in FIG. 2. The series connections may beby vias through the light sheet layers or through switches or couplingsin the external connector 22. A conductor pattern is also formed on thetop substrate 18 for connection to the LED chips' top electrodes. Thecustomizable interconnection of the LED chips allows the drive voltageand current to be selected by the customer or requirements of thedesign. In one embodiment, each identical group of LED chips forms aseries-connected group of LED chips by the conductor pattern and theexternal interconnection of the conductors, and the various groups ofseries connected LED chips may then be connected in parallel to bedriven by a single power supply or driven by separate power supplies forhigh reliability. In yet another embodiment, the LED chips could beformed into a series-parallel connected mesh with additional activecomponents as may be needed to distribute current amongst the LEDs in aprescribed fashion.

In one embodiment, to achieve a series connection of LED chips using topand bottom conductors, some LEDs chips are mounted on the bottomsubstrate with their anodes connected to the bottom substrate electrodesand other LED chips are mounted with their cathodes connected to thebottom electrodes. Ideally, adjacent LED chips are reversely mounted tosimplify the series connection pattern. The conductor between theelectrodes then connects the LED chips in series. A similar conductorpattern on the top substrate connects the cathodes of LED chips to theanodes of adjacent LED chips.

An DC or AC power supply 23 is shown connected to the connector 22. Aninput of the power supply 23 may be connected to the mains voltage. Ifthe voltage drop of an LED series string is sufficiently high, theseries string of LEDs may be driven by a rectified mains voltage (e.g.,120 VAC).

In another embodiment, it is also possible to connect the LED chips intwo anti-parallel series branches, or derivatives thereof, that willenable the LED chips to be driven directly from AC, such as directlyfrom the mains voltage.

FIGS. 3-5, 7, 8, 10-14, and 16-19 are cross-sectional views along line3-3 in FIG. 1, cutting across two LED areas 12, showing the light sheetat various stages of fabrication and various embodiments.

FIG. 3A shows a bottom substrate 14, which may be a commerciallyavailable and customized flex circuit. Any suitable material may beused, including thin metals coated with a dielectric, polymers, glass,or silicones. Kapton™ flex circuits and similar types are commonly usedfor connecting between printed circuit boards or used for mountingelectronic components thereon. The substrate 14 has an electricallyinsulating layer 26, a patterned conductor layer 28, and metalelectrodes 30 extending through the insulating layer 26. The electrodes30 serve as heat sinking vias. Flexible circuits with relatively highvertical thermal conductivities are available. The substrate 14 ispreferably only a few mils thick, such as 1-5 mils (25-125 microns), butmay be thicker (e.g., up to 3 mm) for structural stability. Theconductor layer 28 may be plated copper or aluminum. The electrodes 30are preferably copper for high electrical and thermal conductivity. Theconductor layer 28 may instead be formed on the top surface of thesubstrate 14.

The conductor layer 28 may be any suitable pattern, such as forconnecting the LED chips in series, parallel, or a combination,depending on the desired power supply voltage and current, and dependingon the desired reliability and redundancy.

FIG. 3B illustrates another embodiment of a bottom substrate 32, whichhas a metal reflective layer 34 (e.g., aluminum) sandwiched between atop insulating layer 36 and a bottom insulating layer 38. A conductorlayer 40 and electrodes 42 are formed over the top insulating layer 36.The thickness of the bottom substrate 32 may be 1-5 mils, or thicker,and flexible.

FIG. 3C illustrates another embodiment of a bottom substrate 44, whichhas a dielectric reflective layer 46. This allows the heat conductingmetal electrodes 47 to be formed through the reflective layer 46. Aconductor layer 48 is formed on the bottom of the substrate, but mayinstead be formed on the top surface of the substrate. An optionalinsulating layer 50 overlies the reflective layer 46.

Suitable sheets having a reflective layer may be MIRO IV™, VikuitiDESR™, or other commercially available reflective sheets.

In one embodiment, components of the drive circuitry may be patterneddirectly on the bottom substrate 44 to avoid the need for separatecircuits and PCBs.

FIG. 4 illustrates a conductive adhesive 52, such as epoxy infused withsilver, applied over the electrodes 30. Such a conductive adhesive 52simplifies the LED chip bonding process and increases reliability. Anyof the bottom substrates described herein may be used, and only thebottom substrate 14 of FIG. 3A is used in the examples for simplicity.

FIG. 5 illustrates commercially available, non-packaged blue LED chips56 being affixed to the bottom substrate 14 by a programmedpick-and-place machine or other prescribed die placement method. The LEDchips 56 have a small top electrode 58 (typically used for a wire bond)and a large bottom electrode 60 (typically reflective). Instead of aconductive adhesive 52 (which may be cured by heat or UV) affixing thebottom electrode 60 to the substrate electrode 30, the bottom electrode60 may be ultrasonically welded to the substrate electrode 30, solderreflowed, or bonded in other ways. Suitable GaN LED chips 56 with avertical structure are sold by a variety of manufacturers, such as CreeInc., SemiLEDs, Nichia Inc., and others. Suitable Cree LEDs include EZ290 Gen II, EZ 400 Gen II, EZ Bright II, and others. Suitable SemiLEDsLEDs include the SL-V-B15AK. Such LEDs output blue light, have a toparea of less than about 350×350 microns, and a have a thickness of 170microns. The specifications for some suitable commercially availableblue LEDs, in combination with phosphors to create white light, identifya lumens output in the range of 5-7 lumens per LED at a colortemperature of about 4,100K.

Other types of LED chips are also suitable, such as LED chips that donot have a top metal electrode for a wire bond. Some suitable LED chipsmay have a transparent top electrode or other electrode structures.

FIG. 6 is a perspective view of a transparent intermediate sheet 64having holes 66 for the LED chips 56. Although the LED chips 56themselves may have edges on the order of 0.3 mm, the holes 66 shouldhave a much larger opening, such as 2-5 mm to receive a liquidencapsulant and sufficient phosphor to convert the blue light to whitelight or light with red and green, or yellow, components. The thicknessof the intermediate sheet 64 is approximately the thickness of the LEDchips 56 used, since the intermediate sheet 64 has one function ofpreventing excess downward pressure on the LED chips 56 duringlamination. Transparent sheets formed of a polymer, such as PMMA, orother materials are commercially available in a variety of thicknessesand indices of refraction.

In one embodiment, the bottom surface of the intermediate sheet 64 iscoated with a reflective film (e.g., aluminum) to provide a reflectivesurface. The intermediate sheet may also optionally have a furthercoating of dielectric to prevent electrical contact with traces and toprevent oxidation during storage or handling.

To adhere the intermediate sheet 64 to the bottom substrate 14, thebottom surface of the intermediate sheet 64 may be coated with a verythin layer of silicone or other adhesive material. The silicone mayimprove the total internal reflection (TIR) of the interface byselection of a suitably low index of refraction relative to theintermediate sheet 64.

FIG. 7 illustrates the intermediate sheet 64 having been laminated overthe bottom substrate 14 under pressure. Heat may be used to cure thesilicone. The thickness of the intermediate sheet 64 prevents apotentially damaging downward force on the LED chips 56 duringlamination.

In one embodiment, the intermediate sheet 64 is molded to have prisms 70formed in its bottom surface for reflecting light upward by TIR. If thebottom surface is additionally coated with aluminum, the reflectionefficiency will be improved. Instead of, or in addition to, a prismpattern, the bottom surface may be roughened, or other optical elementsmay be formed to reflect the light through the light output surface.

FIG. 8A illustrates the area 12 surrounding the LED chips 56 filled witha silicone/phosphor mixture 72 to encapsulate the LED chips 56. Themixture 72 comprises phosphor powder in a curable liquid silicone orother carrier material, where the powder has a density to generate thedesired amount of R, G, or Y light components needed to be added to theblue light to create a white light having the desired color temperature.A neutral white light having a color temperature of 3700-5000K ispreferred. The amount/density of phosphor required depends on the widthof the opening surrounding the LED chips 56. One skilled in the art candetermine the proper types and amounts of phosphor to use, such that theproper mixture of blue light passing through the phosphor encapsulantand the converted light achieves the desired white color temperature.The mixture 72 may be determined empirically. Suitable phosphors andsilicones are commercially available. The mixture 72 may be dispensed bysilk screening, or via a syringe, or by any other suitable process. Thedispensing may be performed in a partial vacuum to help remove any airfrom the gap around and under the LED chips 56. The conductive adhesive52 (FIG. 4) helps fill in air gaps beneath the LED chips 56.

In another embodiment, the phosphor around the LED chips 56 in the holesmay be preformed and simply placed in the holes around the LED chips 56.

Instead of the intermediate sheet 64 having holes with straight sides,the sides may be angled or be formed as curved cups such thatreflectance of light outwards is enhanced.

FIG. 8B illustrates the area surrounding the LED chips 56 filled withthe silicone/phosphor mixture 72, where the holes 74 in an intermediatesheet 76 are tapered to reflect light toward the light output surface ofthe light sheet.

All the various examples may be suitably modified if the phosphor isprovided by the LED manufacturer directly on the LED chips 56. If theLED chips 56 are pre-coated with a phosphor, the encapsulant may betransparent silicone or epoxy.

Even if the LED chips 56 are not perfectly centered within a hole 66/74,the increased blue light passing through a thin phosphor encapsulantwill be offset by the decreased blue light passing through the thickerphosphor encapsulant.

FIG. 8C illustrates an intermediate sheet 78 molded to have cups 80surrounding each LED chip 56, where a reflective layer 82 (e.g.,aluminum with an insulating film over it) is formed over the sheet 78 toreflect light toward the light output surface of the light sheet. In theembodiment shown, the cups 80 are filled with a silicone encapsulant 84rather than a silicone/phosphor mixture, since a phosphor tile will belater affixed over the entire cup to convert the blue light to whitelight. In another embodiment, the cups 80 may be filled with asilicone/phosphor mixture.

FIG. 8D illustrates an embodiment where the intermediate sheet 85 isformed of a phosphor or is infused with a phosphor powder, or any otherwavelength conversion material. For example, the intermediate sheet 85may be a molded silicone/phosphor mixture. Since the light generated byphosphor widely scatters, the prisms 70 used in other embodiments maynot be needed.

FIG. 8E illustrates that the LED chips 56 may be pre-coated withphosphor 86 on any sides of the chips, such as on all light-emittingsides or only on the sides and not on the top surface. If the topsurface is not coated with a phosphor, such as to not cover the topelectrode, the blue light emitted from the top surface may be convertedby a remote phosphor overlying the LED chip 56.

FIG. 9 is a perspective view of a transparent top substrate 88 havingelectrodes 90 and a conductor layer 92 formed on its bottom surface. Theelectrodes 90 may be reflective (e.g., silver) or transparent (e.g.,ITO). The top substrate 88 may be any clear flex circuit material,including polymers. The top substrate 88 will typically be on the orderof 1-20 mils thick (25 microns-0.5 mm). Forming electrodes andconductors on flex circuits is well known.

A thin layer of silicone may be silk-screened, sprayed with a mask, orotherwise formed on the bottom surface of the top substrate 88 foraffixing it to the intermediate sheet 64. The electrodes 90 arepreferably not covered by any adhesive in order to make good electricalcontact with the LED chip electrodes 58.

FIG. 10 illustrates a conductive adhesive 94 (e.g., silver particles inepoxy or silicone) dispensed over the top electrodes 58 of the LED chips56.

FIG. 11 illustrates the transparent top substrate 88 laminated over theLED chips 56, using pressure and heat. Heat is optional, depending onthe type of curing needed for the various adhesives. A roller 96 isshown for applying uniform pressure across the light sheet as the lightsheet or roller 96 is moved. Other means for applying pressure may beused, such as a flat plate or air pressure. The thickness of theintermediate sheet 64, matched to the thickness of the LED chips 56,ensures that the laminating force does not exert a pressure on the LEDchips 56 above a damaging threshold. In the preferred embodiment, theforce exerted on the LED chips 56 is substantially zero, since theconductive adhesive 94 is deformable to ensure a good electricalconnection. Further, even if there is some slight protrusion of the LEDchip electrode 58 above the intermediate sheet 64, the elasticity of thetop substrate 88 will absorb the downward laminating pressure.

The thickness of the completed light sheet may be as little as 1-2 mm orless, resulting in little optical absorption and heat absorption. Foradded structural robustness, the light sheet can be made thicker. Ifadditional optics are used, such as certain types of reflecting cups andlight-shaping layers, the total thickness can become up to 1 cm andstill maintain flexibility. The structure is cooled by ambient air flowover its surface. Any of the substrates and intermediate sheetsdescribed herein can be mixed and matched depending on the requirementsof the light sheet.

FIGS. 12A-12D illustrate various phosphor conversion techniques that canbe used to create white light. If UV LED chips are used, an additionalphosphor generating a blue light component would be used.

FIG. 12A illustrates the LED chips' side light being converted to redand green light, or yellow light, or white light and reflected throughthe light output surface of the light sheet, while the blue light fromthe LED chips 56 is directly transmitted through the transparentelectrode 100 on the transparent sheet 88 for mixing with the convertedlight a short distance in front of the light sheet. An observer wouldperceive the light emitted by the light sheet as being substantiallyuniform and white.

FIG. 12B illustrates all the light from the LED chips 56 being emittedfrom the side due to a reflective electrode 104 on the top transparentsheet 88 overlying the LED chips 56. All light is then converted towhite light by the phosphor and reflected through the light outputsurface of the light sheet.

FIG. 12C illustrates side light being converted to white light by thephosphor surrounding the LED chips 56, and the blue top light, emittedthrough the transparent electrode 100, being converted to white light bya remote phosphor layer 106 formed on the top surface of the topsubstrate 88 over the LED chips 56. The phosphor layer 106 may be flator shaped. The area of the phosphor layer 106 is preferable the same asor slightly larger than the LED chips 56. The phosphor layer 106 can berectangular or circular. The phosphor layer 106 is formed such that bluelight passing through the phosphor layer 106 combined with the convertedlight produces white light of the desired color temperature.

FIG. 12D illustrates the LED chips 56 being positioned in reflectivecups 80 filled with a transparent silicone encapsulant, and where sidelight and top light are converted to white light by a large phosphorlayer 108 over each cup 80. In one embodiment, the area of each phosphorlayer 108 is adjusted to allow a selected amount of blue light to bedirectly emitted (not passed through the phosphor) to create the desiredwhite light color temperature. Such phosphor layer sizes can be customtailored at the end of the fabrication process, such as by masking orcutting phosphor tile sizes, to meet a customer's particularrequirements for color temperature.

The top substrate 88 (or any other sheets/substrates described herein)may have a roughened top or bottom surface for increasing the extractionof light and providing a broad spread of light. The roughening may be bymolding, casting, or micro bead blasting.

In another embodiment, shown in FIG. 13, the LED chips 112 may be flipchips, with anode and cathode electrodes 114 on the bottom surface ofthe LED chips 112. In such a case, all conductors 116 and electrodes 118would be on the bottom substrate 120. This would greatly simplify theseries connection between LED chips, since it is simple to design theconductors 116 to connect from a cathode to an anode of adjacent LEDchips 112. Having all electrodes on the bottom substrate 120 alsoimproves the reliability of electrical connections of the substrateelectrodes to the LED electrodes since all bonding may be performedconventionally rather than by the lamination process. The top substrate122 may then be simply a clear foil of any thickness. The top substrate122 may employ the reflectors (from FIG. 12B) above each LED chip 112for causing the chips to only emit side light, or a phosphor layer 124can be positioned on the substrate 122 above each LED chip 112 forconverting the blue light into white light, or any of the other phosphorconversion techniques and intermediate sheets described herein may beused to create white light.

In another embodiment, LED chips are used where both electrodes are onthe top of the chip, where the electrodes are normally used for wirebonding. This is similar to FIG. 13 but where the LEDs are flippedhorizontally and the conductors/electrodes are formed on the topsubstrate 122. The bottom substrate 120 (FIG. 13) may contain metal vias118 for heat sinking, where the vias 118 are bonded to a bottom of theLED chips to provide a thermal path between the LED chips and the metalvia 118 surface exposed on the bottom surface of the bottom substrate.The chips can then be air cooled. A thermally conductive adhesive may beused to adhere the LED chips to the vias 118.

FIG. 14 illustrates LED chips 56 that are alternately mounted on thebottom substrate 14 so that some have their cathode electrodes 60connected to the bottom substrate electrodes 30 and some have theiranode electrodes 58 connected to the bottom substrate electrodes 30. Thetop substrate 88 transparent electrodes 134 then connect to the LEDchips' other electrodes. Since the LED chips' cathode electrode 60 istypically a large reflector, the LED chips connected with their cathodesfacing the light output surface of the light sheet will be sideemitting. The electrodes 30 on the bottom substrate 14 are preferablyreflective to reflect light upward or sideward. The connectors 136 onthe top substrate 88 and the connectors 138 on the bottom substrate 14can then easily connect the adjacent LED chips in series without anyvias or external connections. For converting the top blue light fromsome LED chips to white light, a phosphor layer 142 may be used abovethe LED chips.

FIGS. 15-18 illustrate other embodiments that better enable the LEDchips 56 to be connected in series within the light sheet 10.

FIG. 15 illustrates an intermediate sheet 150 having square holes 152with metal electrodes 154 and 156 formed on opposing walls of the holes152, where the electrode metal wraps around a surface of theintermediate sheet 150 to be contacted by a conductor pattern on thesurface of the intermediate sheet 150 or one or both of the topsubstrate or bottom substrate. The electrodes may be formed by printing,masking and sputtering, sputtering and etching, or by other knownmethods.

As shown in FIG. 16, the LED chips 56, with top and bottom electrodes,are then inserted vertically in the holes 152 so that the LED electrodes58 and 60 contact the opposing electrodes 154 and 156 formed on thewalls of the holes 152. The electrodes 154 and 156 may be first coatedwith a conductive adhesive, such as silver epoxy, to ensure a goodcontact and adhesion. The intermediate sheet 150 has about the samethickness of the chips 56, where the thickness of the chips 56 ismeasured vertically. This helps protect the chips 56 from physicaldamage during lamination.

In the example of FIG. 16, the electrodes 154 and 156 extend to thebottom surface of the intermediate sheet 150 for being interconnected byconductors 158 formed on the bottom substrate 160. In one embodiment,the bottom substrate 160 has a metal reflector layer on its bottomsurface or internal to the substrate for reflecting the side light backup though the light output surface of the light sheet. The reflectivelayer may also be a dielectric layer.

The conductors 158 in FIG. 16 connect the anode of one LED chip 56 tothe cathode of an adjacent LED chip 56. The conductors 158 mayadditionally connect some series strings in parallel (or connectparallel LED chips in series).

FIG. 17 illustrates two light rays generated by the LED chip 56 beingreflected by the LED chip's bottom reflective electrode 60 and thereflective electrode 154 or reflective scattering conductive adhesive.Since the bottom substrate 160 also has a reflector, all light is forcedthrough the top of the light sheet.

Any air gaps between the LED chips 56 and the holes 152 may be filled inwith a suitable encapsulant that improves extraction efficiency.

A phosphor layer 162 converts the blue light to white light.

FIGS. 16 and 17 also represent an embodiment where the conductor patternis formed directly on the bottom or top surface of the intermediatesheet 150, so all electrodes and conductors are formed on theintermediate sheet 150. No top substrate is needed in these embodiments,although one may be desired to seal the LED chips 56.

FIG. 18 illustrates an embodiment where the conductors 166 and 168 areformed on both sides of the intermediate sheet 150 or formed on thetransparent top substrate 170 and bottom substrate 160. The LED chips 56can easily be connected in any combination of series and parallel.

FIGS. 19A and 19B represent an embodiment where the bottom substrate 176has conductors 178 formed on its top surface. The bottom electrodes(e.g., cathodes) of the LED chips 56 are bonded to the conductors 178.For a series connection between LED chips 56, solid metalinterconnectors 180 are also bonded to the conductors 178. Theintermediate sheet 182 has holes that correspond to the LED chip 56locations and interconnector 180 locations, and the tops of the chips 56and interconnectors 180 are approximately planar with the top of theintermediate sheet 182. The areas surrounding the LED chips 56 may befilled in with a phosphor/silicone mixture 72.

In FIG. 19B, a transparent top substrate 184 has anode conductors 186that interconnect the anode electrodes of LED chips 56 to associatedinterconnectors 180 to create a series connection between LED chips 56.This series interconnection technique may connect any number of LEDchips 56 in series in the sheet or strip. A pick and place machine issimply programmed to place an LED chip 56 or an interconnector 180 atselected locations on the bottom substrate 176. The bonding may beperformed by ultrasonic bonding, conductive adhesive, solder reflow, orany other technique.

The interconnector 180 may also be a plating of the hole in theintermediate sheet 182 or a soft conductor paste that is injected intothe hole, printed within the hole, etc.

A phosphor layer or tile 188 may be affixed on the top substrate 184over the LED chips 56 to convert the blue light emitted from the topsurface of the chips 56 to white light. If the phosphor layer/tile 188was large enough, then phosphor need not be used in the encapsulant.

The bottom substrate 176 may have a reflective layer either imbedded init or on its bottom surface, as previously described, for reflectinglight toward the light output surface.

In a related embodiment, the hole for the interconnector may be formedcompletely through the light sheet, then filled with a metal or coatedwith a metal. The hole may be formed using a laser or other means. Themetal may be a printed solder paste that is reflowed to make electricalcontact to the conductors formed on the substrates to complete theseries connection. Extending the metal external to the light sheet willimprove heat sinking to ambient air or to an external heat sinkmaterial. If the metal has a central hole, cooling air may flow throughit to improve heat sinking.

FIGS. 20-31 illustrate various embodiments where there is nointermediate sheet or strip. Instead, the top substrate and/or bottomsubstrate is provided with cavities or grooves to accommodate thethickness of the LED chips 56.

In FIGS. 20A and 20B, the bottom substrate 190 has cavities 192 moldedin it or grooves molded in it. Grooves may also be formed by extruding,machining, or injection molding the substrate 190. The width of thebottom substrate 190 may be sufficient to support one, two, three ormore columns of LED chips 56, where each column of chips 56 is connectedin series, as described below.

Cathode conductors 194 are formed on the bottom substrate 190 and arebonded to the cathode electrodes of the vertical LED chips 56.

A top substrate 196 has anode conductors 198 that are aligned with theanode electrodes of the LED chips 56 and also make contact with thecathode conductors 194 to connect the LED chips 56 in series. The areaaround each LED chip 56 may be filled in with a phosphor/siliconemixture to encapsulate the chips 56, or just silicone may be used as theencapsulant and the top surface of the top substrate 196 is coated witha layer of phosphor to create white light.

FIG. 20B shows the top substrate 196 laminated onto the bottom substrate190. A thin layer of silicone may be printed on the top substrate 196 orbottom substrate 190, except where the conductors are located, to affixthe substrates to each other and to fill in any gaps between the twosubstrates. Alternatively, lamination may be achieved by use of otheradhesive materials, ultrasonic bonding, laser welding, or thermal means.A conductive paste or adhesive may be deposited over the anodeconductors 198 to ensure good electrical contact to the cathodeconductors 194 and chips' anode electrodes. A phosphor tile or layer maybe formed on the top substrate 196 for creating white light from theblue light emitted vertically from the chip 56. A reflective layer 199is formed on the bottom substrate 190 for reflecting light toward theoutput surface.

Instead of the groove or cavity being formed in the bottom substrate190, the groove or cavity may be formed in the top substrate 196, orpartial-depth grooves or cavities may be formed in both substrates toaccount for the thickness of the chips 56.

FIG. 20C is a transparent top down view of FIG. 20B illustrating onepossible conductor pattern for the conductors 194 and 198, where the LEDchips 56 are connected in series, and two sets of series-connected LEDstrings are shown within the laminated substrates. The anode conductors198 above the LED chips 56 are narrow to block a minimum amount oflight. The various metal conductors in all embodiments may be reflectiveso as not to absorb light. Portions of the anode conductors 198 over theLED chips 56 may be transparent conductors.

As shown in FIG. 20D, any number of LED chips 56 may be connected inseries in a strip or sheet, depending on the desired voltage drop. Threeseries strings of LED chips 56 in a single strip or sheet are shown inFIG. 20D, each series string being connected to a controllable currentsource 202 to control the string's brightness. The LED chips 56 areoffset so as to appear to be in a pseudo-random pattern, which isaesthetically pleasing and makes a failed LED chip not noticeable. Ifthere is sufficient diffusion of the light, each string of LED chips maycreate the same light effect as a fluorescent tube. A cathode connectorand an anode connector may extend from each strip or sheet for couplingto a power supply 204 or to another strip or sheet. This allows anyconfiguration of series and parallel LED chips.

In all the embodiments described herein, metal slugs may be providedthat extend through the bottom substrate so as to provide a metal heatpath between the bottom electrodes of the LED chips and air. The slugsmay be similar to the electrodes 30 in FIGS. 3A-5 but may beelectrically insulated from other slugs or electrically connected to theelectrodes of other LEDs by a conductor layer for a series connection. Athin dielectric may separate the LED electrodes from the slugs if theslugs are to be electrically floating.

FIGS. 21A, 21B, and 21C illustrate a different configuration of cathodeconductors 206 and anode conductors 208 on a bottom substrate 210 andtop substrate 212 for connecting the LED chips 56 in series when thesubstrates are brought together. In FIGS. 21A-C, there is only one LEDchip 56 mounted along the width of the structure, and the flexiblestructure can be any length depending on the number of LED chips toconnect in series and the desired distance between LED chips 56. In FIG.21C, the LED chip 56 may be encapsulated in silicone or aphosphor/silicone mixture, and a phosphor tile or phosphor layer 214 isaffixed over the LED chip 56 to generate white light. The phosphor layer214 may be deposited over the entire top surface of the top substrate212. The bottom substrate 210 has a reflective layer 199.

FIG. 22 illustrates that the top substrate 216 may be hemispherical witha phosphor layer 218 over the outer surface of the top substrate 216 forconverting the blue LED light to white light. Silicone encapsulates thechip 56. By providing the top substrate 216 with a rounded surface,there is less TIR and the emitted white light pattern is generallylambertian. Also, for all embodiments, shaping the top substrate can beused to shape the light emission pattern. For example, the top substrateshape can act as a lens to produce a batwing or other non-lambertianemission pattern for more uniform illumination.

The diameters/widths of the substrates in FIGS. 21-22 and the substratesdescribed below may be on the order of 2-10 mm to limit lightattenuation, to maintain high flexibility, to minimize the height of thelight fixture, and to enable handling of the substrates usingconventional equipment. The substrates can, however, be any size.

FIGS. 23A and 23B illustrate that the groove 220 or cavity for the LEDchip 56 may be formed in the top substrate 222 rather than in the bottomsubstrate 224.

In the various embodiments where the LED dies have a semicircular topsubstrate, the light emitted from the dies in the direction of thesubstrate surface less than the critical angle is transmitted throughthe surface. However, light emitted from the dies in the direction ofthe top substrate's length may be subject to more total internalreflection. Therefore, such low angle light or internally reflectedlight should be reflected toward the surface of the top substrate byangled prisms or other reflectors positioned between adjacent LED diesalong the length of the top substrate to provide a uniform emissionpattern along the length of the light strip. The reflectors may beformed in the top or bottom substrates similar to the prisms 70 shown inFIG. 7.

The bottom substrate 224 may be widened to support any number of LEDchips along its width, and a separate hemispherical top substrate 222may be used to cover each separate series string of LED chips mounted onthe single bottom substrate (shown in FIG. 25).

FIG. 24 is a schematic diagram representing that any number of LED chips56 may be connected in a series string 225 in the substrate structure ofFIGS. 21-23.

FIG. 25 illustrates a support base 226 for the separate strings 225 ofLED chips 56. The support base 226 may be a bottom substrate, such assubstrates 210 or 224 in FIGS. 21-23, or may be a separate support basefor strings 225 encased in the top and bottom substrates shown in FIGS.21-23. Each string 225 may be controlled by a separate current source230 and powered by a single power supply voltage connected to the anodesof the strings 225. If some strings output light of a differentchromaticity, or color temperature, the current applied to the variousstrings may be controlled to make the overall chromaticity, or colortemperature, of the light sheet a target chromaticity, or colortemperature. Many driving arrangements are envisioned. In oneembodiment, the support base 226 is nominally 2×4 feet to be areplacement for a 2×4 foot standard ceiling fluorescent fixture. Sinceeach series string 225 of LED chips 56 is very thin, any number ofstrings can be mounted on the support base 226 to generate the requirednumber of lumens to substitute for a standard 2×4 foot fluorescentfixture.

FIGS. 26A-26C illustrate a variation of the invention, where thesubstrates are connected together when initially molded or extruded. Oneor both substrates may be rounded.

In FIG. 26A, a bottom substrate 240 and a top substrate 242 are moldedor extruded together and connected by a resilient narrow portion 244.This allows the top substrate 242 to be closed over the bottom substrate240 and be automatically aligned. Cathode conductors 246 and anodeconductors 248 are formed on the substrates 240 and 242 in thearrangement shown in FIG. 26B so that, when the substrates 240 and 242are brought together, the LED chips 56 are connected in series. Siliconeor a phosphor/silicone mixture may be used to encapsulate the LED chips56, or the outer surface of the substrates is coated with a phosphorlayer to convert the blue light to white light. Any number of LED chips56 can be connected in series within the substrates.

FIG. 26C illustrates the resulting substrate structure affixed to asupport base 250. The support base 250 may have a reflective groove 252for reflecting light 254. The groove 252 may be repeated along the widthof the support base 250 for supporting a plurality of substratestructures.

The bottom substrate 240 may have a flat bottom while the top substrateis hemispherical. This helps mounting the bottom substrate on areflective support base. Providing the top substrate as hemispherical,with an outer phosphor coating, results in less TIR and a morelambertian emission.

In the various embodiments describing overlapping conductors on the topand bottom substrates forming a series connection, the connection may beenhanced by providing solder paste or a conductive adhesive on theconductor surfaces, followed by solder reflow or curing.

FIG. 27 illustrates the use of an LED chip 256 that emits light throughall surfaces of the chip. For example, its cathode electrode may be asmall metal electrode that contacts a transparent (e.g., ITO) currentspreading layer. Such a chip 256 is sandwiched between two substrates258 and 260 that have anode and cathode connectors 262 and 264 thatcontact the chips' electrodes and connect multiple chips in series,similar to the embodiments of FIGS. 20-26.

FIG. 28 illustrates an embodiment where the bottom substrate includes areflective layer 270, such as aluminum, a dielectric layer 272, andconductors 274. The LED chips 56 are in reflective cups 278, such asmolded cups with a thin reflective layer deposited in the cups. The cups278 may be formed in a separate intermediate sheet that is laminatedbefore or after the LED chips 56 are affixed to the bottom substrate.Phosphor 280 fills the area surrounding the LED chips 56. In oneembodiment, the phosphor 280 may fill the entire cup 278 so that the cup278 itself is the mold for the phosphor 280. In another embodiment, someor all of the light-emitting surfaces of the LED chips 56 are coatedwith phosphor 280 prior to the LED chips 56 being affixed on the bottomsubstrate.

The top substrate 282 has conductors 284 that contact the top electrodes58 of the LED chips 56, and the conductors 274 and 284 may come incontact with each other using the various techniques described herein toconnect the LED chips 56 in series. The top substrate 282 has formed onits surface a phosphor layer 286 that converts the LED chips'top-emitted light to white light. The top substrate 282 may have anoptical layer 288 laminated over it. The optical layer 288 has a pattern290 molded into it that is used to create any light emission patterndesired. The pattern 290 can be a Fresnel lens, diffuser, collimator, orany other pattern.

In one embodiment, the bottom substrate of FIG. 28 is 1-2 mm thick, thecup layer is 2-3 mm, the top substrate 282 is 1-2 mm, and the opticallayer 288 is 2-3 mm, making the overall thickness about 0.6-1 cm.

FIG. 29 illustrates a portion of a light sheet with a repeating patternof strings of LED chips 56. The view of FIG. 29 is looking into an endof a series string of LED chips 56. A bottom substrate 292 includes areflective layer 294 and a dielectric layer 296. Conductors 298 areformed on the dielectric layer 296, and LED chip electrodes areelectrically connected to the conductors 298.

A top substrate 300 has cavities or grooves 302 that extend into theplane of FIG. 29 and contain many LED chips 56 along the length of thelight sheet. If the top substrate 300 extends across the entire lightsheet, there would be many straight or meandering grooves 302, where thenumber of grooves depends on the number of LED chips used. The topsubstrate 300 has conductors 304 that contact the top electrodes of theLED chips 56 and contact the conductors 298 on the bottom substrate 292for creating series strings of LED chips 56 extending into the plane ofFIG. 29. The series strings and light sheet structure may resemble thatof FIG. 25, having an integral top substrate extending across the entiresheet. The conductors 304 may be transparent directly above the LEDchips 56.

The portions of the top substrate 300 directly over the LED chips 56have a phosphor coating 306 for generating white light. The topsubstrate 300 is molded to have reflecting walls 308 along the length ofthe string of LED chips to direct light outward to avoid internalreflections. The reflective walls 308 may have a thin metal layer. Thetop and bottom substrates may extend across an entire 2×4 foot lightsheet. Alternatively, there may be a separate top substrate for eachstring of LED chips 56.

At the end of each series string of LED chips or at other points in thelight sheet, the anode and cathode conductors on the substrates must beable to be electrically contacted for connection to a current source orto another string of LED chips, whether for a series or parallelconnection. FIGS. 30A, 30B, and 31 illustrate some of the many ways toelectrically connect to the various conductors on the substrates.

FIG. 30A illustrates an end of a sheet or strip where the bottomsubstrate 310 extends beyond the top substrate 312, and the ends ofconductor 314 and 315 on the bottom substrate 310 are exposed. Substrate310 is formed of a reflective layer 311 and a dielectric layer 313. FIG.30B is a top down view of the end conductors 314 and 315 on the bottomsubstrate 310 and an end conductor 316 on the top substrate 312. Theconductor 316 contacts the anode electrode of the LED chip 56 andcontacts the conductor 315.

The ends of the exposed portions of the conductors 314 and 315 arethickly plated with copper, gold, silver, or other suitable material toprovide connection pads 317 for solder bonding or for any other form ofconnector (e.g., a resilient clip connector) to electrically connect theanode and cathode of the end LED chip 56 to another string or to a powersupply. The connection pads 317 may be electrically connected to aconnector similar to the connector 22 in FIG. 2 so the connections toand between the various strings of LED chips 56 can be determined by thecustomized wiring of the connector 22 to customize the light sheet for aparticular power supply.

FIG. 31 is a side view of a portion of a light sheet showing platedconnection pads 318-321 formed along the bottom substrate 324 that leadto conductors, such as conductors 314 in FIG. 30A, on the bottomsubstrate 324. Pads 318 and 319 may connect to the anode and cathodeelectrodes of an LED chip at the end of one string of LED chips, andpads 320 and 321 may connect to the anode and cathode electrodes of anLED chip at the end of another string of LED chips. These pads 318-321may be suitably connected to each other to connect the strings in seriesor parallel, or the strings may be connected to power supply terminals.In one embodiment, a string of LED chips consists of 18 LED chips todrop approximately 60 volts. The pads 318-321 may act as surface mountedleads soldered to a conductor pattern on a support base, since solderwill wick up on the pads 318-321 while soldering to the conductorpattern. The pads 318-321 may also be connected using a resilient clipconnector or other means. The pads 318-321 may also extend to the bottomsurface of the substrate 324 for a surface mount connection.

In the various embodiments, the material for the substrates preferablyhas a relatively high thermal conductivity to sink heat from the lowpower LED chips. The bottom substrates may even be formed of aluminumwith a dielectric between the conductors and the aluminum. The aluminummay be the reflector 199 in FIG. 20A or other figures. The backplane onwhich the LED/substrates are affixed may be thermally conductive.

The various conductors on the transparent top substrates may be metaluntil proximate to each LED chip, then the conductors become atransparent conductor (e.g., ITO) directly over the LED chip to notblock light. A conductive adhesive (e.g., containing silver) may be usedto bond the LED chips' anode electrode to the ITO.

The wavelength converting material, such as phosphor, can be infused inthe top substrate, or coated on the top substrate, or used in the LEDchip's encapsulant, or deposited directly over the LED chip itself, orformed as a tile over the LED, or applied in other ways.

The LED chips/substrate structures may be mounted on any suitablebackplane that may include reflective grooves in a straight ormeandering path. It is preferable that the LED chips appear to be in apseudo-random pattern since, if an LED chip fails (typically shorts), itwill not be noticeable to a viewer.

The top substrate may be molded with any optical pattern to shape thelight emission. Such patterns include Fresnel lenses or holographicmicrostructures. Also, or instead, an additional optical sheet may bepositioned in front of the substrate structures for shaping the light,such as diffusing the light, to meet the requirements of office lightingdirected by the Illuminating Engineering Society of North America,Recommended Practice 1-Office Lighting (IESNA-RP1).

In addition, having a plurality of strips of LED chips, with the stripshaving different optical structures for different light emissionpatterns, could be used with a controller that controls the brightnessof each strip to create a variable photometric output.

The number of LED chips, chip density, drive current, and electricalconnections may be calculated to provide the desired parameters fortotal flux, emission shape, and drive efficiency, such as for creating asolid state light fixture to replace standard 2×4 foot fluorescentfixtures containing 2, 3, or 4 fluorescent lamps.

Since the substrates may be only a few millimeters thick, the resultingsolid state luminaire may be less than 1 cm thick. This has greatadvantages when there is no drop ceiling or in other situations wherespace above the luminaire is limited or a narrow space is desirable.

In embodiments where there is a conductor over the LED chip, a phosphorlayer may be deposited on the inside surface of the substrate followedby an ITO deposition over the phosphor so that LED light passes throughthe ITO then excites the phosphor.

To avoid side light from the LED chips becoming scattered in thesubstrates and attenuated, 45 degree reflectors, such as prisms, may bemolded into the bottom substrate surrounding each LED chip, similar tothe prisms 70 in FIG. 7, to reflect light toward the light outputsurface of the light sheet.

Since the substrates are flexible, they may be bent in circles or arcsto provide desired light emission patterns.

Although adhesives have been describe to seal the substrates together,laser energy, or ultrasonic energy may also be used if the materials aresuitable.

It is known that LED chips, even from the same wafer, have a variety ofpeak wavelengths so are binned according to their tested peakwavelength. This reduces the effective yield if it is desired that thelight sheet have a uniform color temperature. However, by adjusting thephosphor density or thickness over the various LED chips used in thelight sheet, many differently binned LED chips can be used whileachieving the same color temperature for each white light emission.

The LEDs used in the light sheet may be conventional LEDs or may be anytype of semiconductor light emitting device such as laser diodes, etc.Work is being done on developing solid state devices where the chips arenot diodes, and the present invention includes such devices as well.

Quantum dots are available for converting blue light to white light (thequantum dots add yellow or red and green components to create whitelight). Suitable quantum dots can be used instead of or in addition tothe phosphors described herein to create white light.

To provide high color rendering, the direct emissions of LED chips inthe light sheet emitting red and green light can be controlled to mixwith the white light emitted by phosphor-converted LED chips to producea composite light that achieves high color rendering and enables thepossibility of tuning the light by independent or dependent control ofthe red and green LEDs by open loop deterministic means or closed loopfeedback means or any combination thereof. In one embodiment, differentstrings of LED chips have different combinations of the red, green, andphosphor-converted LEDs, and the strings are controlled to provide thedesired overall color temperature and color rendering.

Since the light sheet is highly flexible and extremely light, it may beretained in a particular shape, such as flat or arced, using alight-weight frame.

FIG. 32 is a perspective view of a plastic frame 330 for supporting theflexible light sheet strip or sheet 10 by its edges or over otherportions of its surface (depending on the width of the light sheet) toselectively direct light toward an area directly under the light sheet.Other configurations are achievable. Thin sheets containing opticalelements for further control of the light emission from the light sheetmay be supported by the frame 330.

In some applications, it may be desirable to have a luminaire emit lightgenerally downward and off the ceiling for a certain lighting effect.Accordingly, all the light sheet/strip embodiments may be adapted tocreate a bidirectional sheet or strip.

Multiple light sheets may also be mounted in a ceiling fixture as flatstrips, and each strip is tilted at a different angled relative to thefloor so that the peak intensities of the strips are at differentangles. In one embodiment, the peak intensity is normal to the flatsurface of the light sheet, assuming no re-directing lenses are formedin the light sheet. Therefore, the shape of the light pattern from thefixture can be customized for any environment and can be made to mergewith light from other fixtures. In one embodiment, some light strips areangled downward at 55 degrees, and other light sheets are angled upwardto reflect light off the ceiling.

FIG. 33 illustrates LED dies 56 that are oppositely mounted in a lightsheet to create a bidirectional emission pattern. This is similar toFIG. 14, but there is no reflector covering the entire bottom substrate.In FIG. 33, any number of LED dies 56 are connected in series byalternating the orientation of the LED dies along the light sheet toconnect the anode of one LED die to the cathode of an adjacent LED dieusing metal conductors 340 and 342 formed on the top substrate 344 andbottom substrate 346. The substrate electrodes contacting the LEDelectrodes 58, formed on the light-emitting surface of the LED dies, maybe transparent electrodes 348 such as ITO (indium-doped tin oxide) orATO (antimony-doped tin oxide) layers. A phosphor layer 350 may bedeposited to generate white light from the blue LED emission.

FIG. 34 illustrates two light sheets back-to-back, similar to the lightsheet of FIG. 13, but sharing a common middle substrate layer 351. TheLED dies 352 are shown as flip-chips, and the conductor layers forinterconnecting the LED dies on each side in series are deposited onopposite sides of the middle substrate 351. The light sheet structure issandwiched by transparent substrates 356 and 358. The middle substrate351 may include a reflective layer that reflects all impinging lightback through the two opposite surfaces of the bidirectional light sheet.

FIG. 35 is another example of two light sheets or strips, similar to thelight sheet described with respect to FIG. 20B, affixed back-to-backwith a middle reflective layer 360. The conductors 194 and 198 andsubstrates 196 and 190 are described with respect to FIGS. 20A and 20B.The light sheets may be affixed to the middle reflective layer 360 usinga thin layer of silicone or other adhesive. Phosphor (not shown) may beused to convert the blue LED light to white light.

The middle reflective layer 360 may have as a property that it is a goodconductor of thermal energy which can assist the traces 194 indissipating the heat from the chips 56. There may be enough thermal masswithin the middle layer 360 that it provides all of the heat sinkrequired to operate the chips safely or it may be extended laterally(beyond the edges of the substrates 190 and 196, shown in dashedoutline) to regions where the heat may be dissipated more freely to theair within the lighting fixture.

Any of the light sheet/strip structures described herein may be adaptedto create a bidirectional light sheet.

The light output surfaces of the various substrates may be molded tohave lenses, such as Fresnel lenses, that customize the light emissionpattern, such as directing the peak intensity light 55 degrees off thenormal, which is a desired angle to reduce glare and to allow the lightto merge smoothly with light from an adjacent fixture. Different lensesmay be formed over different LED dies to precisely control the lightemission so as to create any spread of light with selectable peakintensity angle(s).

FIG. 36 illustrates a bidirectional light sheet 362 hanging from aceiling 364. Light rays 366 are shown being reflected off the ceilingfor a soft lighting effect, while downward lighting provides directlight for illumination. The light output surfaces of the light sheet 362may be patterned with lenses, as described above, to create the desiredeffect. The top and bottom light emissions may be different to achievedifferent effects. For example, it would be desirable for the upwardemitting light sheet to output the peak light emission at a wide angleto achieve more uniform lighting of the relatively proximate ceiling,while the downward emitting light sheet would emit light within anarrower range to avoid glare and cause the light to smoothly merge withlight from an adjacent fixture. In one embodiment, the size of the lightsheet 362 is 2×4 feet; however, the light sheet 362 can be any size orshape.

The top and bottom light emissions may also be adapted to have differentspectral contents in addition to different optical dispersioncharacteristics. It is advantageous in some designs to consider that thesoft fill light from above have one spectral content such as the lighterblue of daylight, for example 5600 Kelvin, and the direct lightdownwards having a preferred spectral content such as 3500 Kelvin, whichmimics direct sunlight. The design of light sheet 362 is well suited tothe creation of these two components. Furthermore, the modulation oflight levels from the top and bottom light emissions may differtemporally as in the simulation of a day lighting cycle or to favorbackground illumination over direct illumination or in any combinationas may be desired by users to increase their comfort and performance oftasks within the space.

Alternatively, the bidirectional light sheet 362 may be mounted in aconventional diffusively reflective troffer.

In one embodiment, the ceiling panels above the fixture may be infusedwith phosphor or other wavelength conversion material to achieve adesired white point from the ceiling light. In such a case, the lightsheet may direct UV or blue light toward the ceiling.

In some applications, it may be desirable to provide a bidirectionallight sheet emitting low intensity up-light and higher intensity downlight, or vice versa. In the various disclosed embodiments ofunidirectional light sheets having a reflective layer, the reflectivelayer may be omitted so there is a primary light emission surface and anopposing light leakage surface. The light leakage may be useful incertain applications, such as illuminating a ceiling to avoid a shadowand decreasing luminance contrast ratios.

To avoid any manufacturing difficulties with lamination and alignment,the snap-in structure of FIG. 37A may be used. The LED die 368 ismounted on a trapezoidal or frustum shaped base substrate 370. The basesubstrate 370 can have many other shapes that mate with a correspondingmating feature in a top substrate 372. The base substrate 370 can besmall and support a single LED die 368 or may be a strip and supportmany LED dies (e.g., 18) connected in series. The conductor 374 connectsto the die's top electrode 376, and the conductor 378 connects to thedie's bottom electrode 380 via the base substrate's conductor 382. Theconductors 374 and 378 extend into the plane of the figure to create aseries connection between adjacent LED dies (anode to cathode) along thelength of the top substrate 372 one example of which is shown in FIG.37B.

As seen in FIG. 37B, the conductor 374 connected to the LED's top (e.g.,anode) electrode 376 leads to the conductor 378 connected to theadjacent LED's bottom (e.g., cathode) electrode. The serpentine patterncontinues to connect any number of LEDs together. Many other conductorpatterns may be used to make the series connections. Alternatively, theconductor pattern used make the series connections may be formed on thesnap-in strips (supporting the LED dies 368).

At least the top substrate 372 is formed of a resilient material, suchas transparent plastic or silicone, so as to receive the base substrate370 and resilient fix it in place. The spring force will provide areliable compressive force between the opposing conductors, so aconductive adhesive between the abutting metal surfaces may be optional.The resulting structure may contain a string of LED dies that can bemounted on a larger support substrate with other strings of LED dies, orthe top substrate 372 may extend laterally to receive multiple strips ofbase substrates 370, each supporting a series string of LED dies. Theresulting structure may resemble that of FIG. 25, where the substratescan be any length and contain any number of LED dies. FIG. 37A shows thereplication of identical top substrates 372 as part of a single largesubstrate. The top substrate 372 may be molded to have side reflectors384 coated with a reflector or with a diffused reflector. Thehalf-cylindrical top surface of the top substrate 372 may have aphosphor layer 386 for generating white light. A remote optical sheet388 may be molded with optical elements (e.g., prisms, lenses, etc.) tocreate any light emission pattern.

In one embodiment, the base substrate 370 is formed of a metal, such asaluminum, with a dielectric coating so that the base substrate 370 actsas a heat sink. Since the back surface of the base substrate 370 will bethe highest part of the light sheet/strip when the light sheet ismounted in a ceiling or fixture, ambient air will cool the exposedsurface of the metal.

In the various snap-in embodiments, the top substrate may be flexed toopen up the edges of the receiving cavity or groove to allow the diesubstrate to easily snap in place. Alternatively, the top substrate maybe heated to the point of plastic deformation such that the diesubstrate could also be readily inserted and the assembly then allowedto cool thereby locking the two parts together.

An encapsulant may be deposited along the sides of the die, which thensquishes out when the die substrate snaps in place to encapsulate thedie and provide a good index of refraction interface between the die andthe top substrate.

The die substrates may be formed as a strip, supporting a plurality ofspaced dies, or may be formed to only support a single die.

FIG. 38 illustrates how a plurality of top substrates 372 may be snappedover mating features of a single bottom substrate 392 that is molded tocreate islands or strips of snap-in features 394, similar to thosedescribed with respect to FIG. 37A. Using such snap-in techniquesautomatically aligns the top and bottom substrates and simplifies theelectrical contacts for forming serial strings of LEDs. The conductorpattern of FIG. 37B may be used with all the snap-in embodiments toconnect the LED dies in series.

The phosphor layer 386 may be different for each serial column of LEDchips so that the overall color temperature of the light sheet can beadjusted by changing the brightness of the various series strings of LEDchips. For example, a thinner phosphor layer 386 will create bluerlight, and the brightness of the associated LED chips can be adjusted tomake the overall color temperature higher or lower. Many variations canbe envisioned where different chromaticity of each LED string phosphorlayer 386 may be controlled to create tunable white light.

In one embodiment, the bottom substrate 392 is formed of one type ofmaterial, such as a dielectric, and the snap-in features 394 may be diesubstrates formed of a different material, such as metal.

FIG. 39 illustrates that the bottom substrate 396 may include one ormore curved reflectors 398 along the length of the LED strip to reflectside light toward an object to be illuminated. The reflectors 398 may bepart of a molded, single piece substrate 396. A reflective film may bedeposited over the curved surface. The top substrate 400, resembling ahalf cylinder, snaps over the mating feature of the bottom substrate 396and can be any length.

The top or bottom substrate in FIGS. 37A-39 may be formed withadditional reflectors, such as prisms (previously described), thatreflect an LED die's light toward the output surface when the light isemitted into and out of the plane of the figures. In addition, moldedvariations in the outer profile of the top substrate 400 in thelongitudinal direction may be advantageous to increase light emissionout of the top substrate out of the plane of the figures. Phosphor layer386 on the top substrate may be a layer of any wavelength convertingmaterial which can alter the final emitted light spectrum from thedevice. There can be variations in the density and thickness of thiscoating to achieve a desirable spatial emitting pattern of lightspectrum.

FIG. 40 is similar to FIG. 37A except that the LED die substrate 410 isfixed in place by a conductive adhesive 412 or solder reflow. There areno snap-in features in FIG. 40. Pressing the substrate 410 into the topsubstrate 414 causes the conductive adhesive 412 to make electricalcontact with the conductors 374 and 378. Curing the conductive adhesive412, such as by heat, UV, or chemical catalyst action creates a bond.

If required for heat sinking, the LED die substrate 410 may include ametal slug 416 for transmitting heat to the ambient air, or the diesubstrate 410 itself may be metal.

In all embodiments of a light sheet with a phosphor overlying the LEDchips, the LED chips may first be energized and tested for colortemperature and brightness before or after being part of the lightsheet. Then, each phosphor tile or layer deposited on the top substrateover an associated LED chip can be customized for the particular LEDchip to achieve a target white point. In this way, there will be coloruniformity across the surface of the light sheet irrespective of thepeak wavelength of the individual blue LED chips. However, even if thesame phosphor tile were positioned over each LED chip, the large numberof LED chips (e.g., 300-600) would ensure that the overall (averaged)emitted light from the light sheet will be consistent from one lightsheet to another in the far field.

FIG. 41 illustrates a small portion of a bidirectional light sheet 420,similar to FIG. 35, positioned in front of an air vent 424 in a ceiling425, where UV LED chips 426 are mounted in the upper portion, and blueLED chips 428 (along with phosphor) are mounted in the bottom portion.The top emission is UV for disinfecting air 430, and the bottom emissionis white light for illumination. The direction of air flow around thefixture can either be from the ceiling down or it could be part of thereturn air path where the air flows upwards and around the fixture whereit is recycled and re-used in the space.

FIG. 42 is similar to FIG. 41 but the air 440 is allowed to flow throughholes 441 in the light sheet 442 and/or forced around the edges of thelight sheet 442. The light sheet 442 may be installed as a ceilingpanel. More specifically, FIG. 41 illustrates a small portion of abidirectional light sheet 442 positioned in front of an air vent or airreturn duct in a ceiling, where UV LED chips 426 are mounted in theupper portion, and blue LED chips 428 (along with phosphor) are mountedin the bottom portion. The top emission is UV for disinfecting air 440,and the bottom emission is white light for illumination.

If a phosphor layer is positioned over an LED chip, the phosphor layershould ideally intercept all the blue light emitted from the LED chip.However, due to light spreading in the transparent top substrate, theblue light may spread beyond the edges of the phosphor layer, creatingan undesirable blue halo. FIG. 43, similar to FIG. 20B, illustrates howa lens 446 may be formed (e.g., molded) in the top substrate 448 on thesurface opposing the LED chip 450. In one embodiment, the lens 446 is aFresnel lens. The lens 446 serves to collimate the LED light 452 so thata larger percentage of the blue light impinges on the phosphor tile 454.This will avoid a blue halo around each LED area. A lens in the topsubstrate may be employed for other purposes to create any lightemission pattern.

Although the examples of the light sheets herein have used blue LEDchips with phosphors or other wavelength conversion materials (e.g.,quantum dots) to create white light, white light may also be created bymixing the light from red, green, and blue LED chips, as shown in FIG.44. FIG. 44 illustrates that red LED chips 456, green LED chips 457, andblue LED chips 458 may make up the light sheet 460 (similar to that ofFIG. 20B) and be controllable to achieve any white point. Othercombinations of LED chips and phosphor converted LED chips, orassemblies, can also be combined in numerous ways to produce differentpossible gamuts of light that can be controlled to produce specificcolor and white points.

The LED chips of a single color may be connected in series, and therelative brightness of the strings of LED chips is controlled by currentto achieve the desired overall color or white point of the light sheet.

In another embodiment, various strings of LED chips may bephosphor-converted chips producing white light. Other strings may becomposed of LED chips producing red, green, or blue light to allow thosestrings to be controlled to add more red, green, or blue to the whitelight.

Alternatively, all blue or UV LED chips may be used but the phosphorsmay be selected for each LED area to generate either red, green, or bluelight. The relative brightness of the red, green, and blue light may becontrolled to generate any overall color or white point.

FIG. 45, similar to FIG. 20B, illustrates that blue and infrared LEDchips may make up the light sheet 470, where blue LED chips 458 are usedfor generating white light in conjunction with some form of wavelengthconverting material, and the infrared LED chips 472 are only energizedwhile the blue LEDs are off, such as in response to a motion sensor, forproviding low energy lighting for surveillance cameras. No phosphor isused with the IR chips. It is known to generate IR light by dedicatedfixtures for surveillance camera illumination, but incorporating IR LEDchips in light sheet fixtures that contain other chips for producingwhite light for general illumination of a room is an improvement andcreates synergy, since the locations of the white light fixtures ensurethat the IR light will fully illuminate the room.

Various light sheet embodiments disclosed herein have employedconductors on the inner surfaces of the top and bottom substratesopposing the LED chip electrodes. FIGS. 46A and 46B illustrate atechnique where the conductors are formed on the outside surface of thesubstrates for possible improvement in electrical reliability and heatsinking FIG. 46A illustrates masked or focused laser light 480 ablatingopenings 484 in a top substrate 486 and a bottom substrate 488 of alight sheet 489 for exposing the top and bottom electrodes of LED chips490. Also, areas of the light sheet may be completely ablated throughfor forming a series connection. The laser may be an excimer laser. Areflector layer 492 is also shown. FIG. 46A may also be formed byplastic deforming two plastic substrate layers such that LED chips 490are encased between the two sheets of material 488 and 486 under thecorrect temperature and pressure. Once encased, their top and bottomcontacts are exposed by laser removal of materials down to theelectrical contact points on the die.

In FIG. 46B, a metal 494, such as copper or aluminum, or a conductivemetallic composite material, fills the openings 484 to electricallycontact the LED chips' electrodes. Metal deposition may be by printing,sputtering, or other suitable technique. If a phosphor layer is used,the phosphor may be deposited before or after the laser ablation andbefore or after the metal deposition. In the example, the metal 494fills the openings 484 and also forms a conductor pattern that connectsany number of LED chips in series. The metal contacting the bottomelectrode of the LED chips will also sink heat since it will be facingupward when the light sheet is installed as a fixture.

Some blue LED chips, such as the SemiLEDs SL-V-B15AK vertical LED, areextremely thin, so there is minimal side light and high extractionefficiency. The thickness of the SL-V-B15AK die is only about 80microns, which is less than a typical sheet of paper (about 100microns). The bottom surface area of the SL-V-B15AK is about 400×400microns. The data sheet for the SL-V-B15AK is incorporated herein byreference. In one embodiment of a light sheet to replace a standard 2×4foot fluorescent lamp troffer, there are about 500 LED chips, with anaverage pitch of about 2 inches (5 cm). By using such thin LED chips,the flexibility and plasticity of the substrates allows the substratesto seal around the LED chips, obviating the need for any cavity, groove,or intermediate layer to accommodate the thickness of the LED chip. Anencapsulant may be unnecessary for light extraction if there is directcontact between the top substrate and the top surface of the LED chip.

FIGS. 47A-47C illustrate sandwiching a thin LED chip 500 between twosubstrates 502 and 504 without the use of any cavity, groove, orintermediate layer to accommodate the thickness of the LED chip 500. Thebottom substrate 502 has a conductor pattern 506 with an electrode 507for bonding to the nominal wire bond electrode 508 of the LED chip 500.A typical conductor (a metal trace) thickness is less than 35 microns. Asmall amount of a conductive adhesive 510 (e.g., silver epoxy) may bedeposited on the electrode 507. The electrode 507 may be a transparentlayer, such as ITO. An automatic pick and place machine uses machinevision to align the LED chip 500 with a fiducial formed in the conductorpattern 506. A typical placement tolerance for such pick and placemachines is on the order of 20 microns. The LED chip electrode 508 has awidth of about 100 microns, so bonding the electrode 508 to thesubstrate electrode 507 is a simple task.

A very thin layer of silicone may be printed on the surface of thebottom substrate 502 as an adhesive and to seal around the LED chip 500.

Next, the top substrate 504 is laminated over the bottom substrate 502.The top substrate 504 has a conductor pattern 520 that makes electricalcontact with the LED chip bottom electrode and the conductor pattern 506on the bottom substrate to create a serial connection between LED chips.A small amount of conductive adhesive 522 is deposited on the conductorpattern 520 to ensure good electrical contact. FIG. 47B illustrates asimplified portion of the laminated light sheet; however, in an actualdevice, the top and bottom substrates (along with any thin siliconelayer) will conform to the LED chip 500 and bend around it to seal thechip.

FIG. 47C is a bottom up view of FIG. 47B illustrating the seriesconnections between LED chips. Many other conductor patterns may be usedto create the serial connection.

FIG. 48 is a perspective view of a solid state lighting structure 604that can directly replace standard fluorescent lamps in fixtures as aretrofit for reducing energy consumption and adding controllability. Alight strip 606, representing any of the embodiments described herein,is supported by any means between two sets of standard fluorescent lampelectrodes 608 (or suitable facsimiles) that provide drive power to theLED dies on the strip 606. In one embodiment, the light strip 606 isbidirectional. The electrodes 608 will typically provide the onlyphysical support of the structure 604 within the fixture. In anothertype of fixture, the structure 604 may be additionally supported alongits length by a support attached to the fixture. The electrodes 608 mayprovide a non-converted mains voltage to a converter on the strip 606 orin a separate module. It is preferable that the driver convert the mainsvoltage to a higher frequency or DC voltage to avoid flicker. Driversfor strings of LED dies are commercially available. Alternatively, theconverter may be external to the structure 604 so the electrodes 608receive the converted voltage. Further, the structure 604 may also beadapted to work with the standard output from the retrofit fluorescentballast. Air vents may be made along the structure 604 to remove heat.In one embodiment, the light strip 606 is within a transparent ordiffused plastic, or glass, tube for structural integrity. The tube mayalso have optical characteristics for mixing and shaping the light.

Any number of light strips 606 may be supported between the electrodes608, and the light strips 606 may have different emission patterns orangles. For example, some light strips 606 may emit a peak intensity at55 degrees relative to the normal, while others may emit a peakintensity at 0 degrees. The brightness of each strip 606 may becontrolled to provide the desired overall light emission for thestructure 604. In one embodiment, the structure 604 is about four feetlong.

It is further advantageous to recognize that the US Department of Energyin their testing has noted that many of the commercially availablefluorescent type replacement products utilizing LED sources fail tointeract correctly with the fixture and produce the incorrectillumination patterns or create undesirable glare that is outside theaccepted practice known as RP1. It is another object of the invention toadapt the optics of the sheet within the tube so that it provides a morefavorable distribution of light from the light fixture.

The planar light sheet 606 may be pivotally suspended from and connectedbetween two ends of the outer tube structure 604 by means of a pivotjoint 609. This allows the light sheet 606 to be turned such that itstop and bottom faces may be presented in any orientation within thelight fixture once the electrodes are mechanically locked and energized.This ability to orient the light sheet independent of the ends providesa means for the installation and commissioning staff to adjust the lightdistribution within the fixture to suit user preference or to complywith field lighting requirements. Since the tube can have openings, itis an easy task to insert a tool through a hole to tilt the light sheet606.

In another embodiment, the outer tube of the structure 604 iseliminated, and the light strip 606 is supported by the electrodes 608.This improves heat and light extraction. If required, the light strip606 may be supported by an additional support rod or platform betweenthe electrodes 608.

FIG. 49 illustrates how the fluorescent tube form factor may be changedto have a flat surface 610 that supports the light strip 606 (FIG. 48)and improves heat transfer to the ambient air. When the structure 612 ofFIG. 49 is mounted in a fixture, the flat surface 610 will be at thehighest point to allow heat to rise away from the structure. Air vents614 may be formed in the flat surface 610 and through the light sheet,if necessary, to allow heated air to escape. The flat surface 610 mayalso have patterns of corrugations at the same or different scales(e.g., wide/deep and narrow/shallow) to enhance heat dissipation. Sinceonly low power LED dies are used in the light sheet, and the heat isspread over virtually the entire area of the light sheet, no specialmetal heat sinks may be needed, so the structure 612 is light weight,comparable to a standard fluorescent lamp. This may allow the structure612 to be supported by the electrode sockets in a standard fixture. Insome embodiments, the structure 612 may be lighter than a fluorescenttube, since the structure is only half a cylinder and the tube materialcan be any thickness and weight.

In another embodiment, the flat surface 610 may be a thermallyconductive thin sheet of aluminum for spreading heat. The light strip606 may include metal vias distributed throughout it and thermallyconnected to the sheet of aluminum to provide good heat sinking from theLED chips. The aluminum sheet may also add structural stability to thelight strip 606 or structure 612.

FIG. 50 is a cross-sectional view of a fixture 616 incorporating thelight structure 612 of FIG. 49, with the light strip 606 being supportedby the top flat surface 610 of the structure 612 and showing heated air618 escaping through vents 614 in the flat surface 610 and throughcorresponding holes (not shown) in the LED strip 606. LED dies 624 areshown. A voltage converter 622 is shown internal to the structure 612,but it may be external.

In the example of FIG. 50, there are three different light strips orportions 626, 627, and 628, each having a different peak light intensityangle to allow the user to customize the light output of the fixture616. Three light rays 629, 630, and 631, representing the different peaklight intensity angles, illustrate that the different light strips orportions 626-628 have different emission properties. The light emissionof a strip may be customized by the angles of reflectors making up thestrip, or external to the strip, or lenses molded into the top surfacesof the strips.

FIG. 51 is a side view of an embodiment where the flexible light sheet650 is bent to have a tube shape to emulate the emission of afluorescent tube. The light sheet 650 may be any of the embodimentsdescribed herein. The light sheet 650 may have holes 652 to allow heatto escape. End caps 654 interface the light sheet 650 to the standardelectrodes 656 typically used by fluorescent tubes. A supporting rod maybe incorporated in the middle between the caps 654 to provide mechanicalsupport for the structure.

A larger, substantially cylindrical structure, but without theprotruding electrodes 656, may instead be suspended from a ceiling as astandalone fixture. Such a fixture will illuminate the ceiling and floorof a room.

FIG. 52 is a perspective view of a light sheet 670, illustrating that abidirectional light sheet may be bent to have a rounded shape. The topemission 672 may illuminate a ceiling, and the bottom emission 674 willbroadly illuminate a room. The orientation of the bidirectional lightsheet 670 may be reversed to provide more directed light downward.

FIG. 53 is a perspective view of a fixture 678 that includes abidirectional light sheet 680 suspended from a top panel 682 by wires683. The top emission 684 impinges upon the top panel 682, where the toppanel may be diffusively reflective or have a phosphor coating that canconvert the upward blue light from the LED chips to substantially whitelight. Some ratio of the blue light may be reflected and some may beabsorbed and converted by the light converting material in the panelsuch that the composite light output is substantially white light thatis emitted into the room. The top panel 682 will typically be muchlarger than the light sheet 680. This may be appropriate for a lightfixture for very high ceilings, where the fixture is hung relatively farfrom the ceiling. If the top panel 682 is coated with a phosphor,interesting lighting and color effects may be created. The top emission684 may be blue or UV light, and the bottom emission 686 may be whitelight. The top panel 682 may be any shape, such as a gull wing shape orV shape to direct light outward.

In the various embodiments, the phosphor, whether infused in the topsubstrate or a separate layer, may be varied to take into account thehigher blue light intensity directly over the LED chip compared to theintensity at an angle with respect to the chip. For example, thephosphor thickness or density may be tapered as the phosphor extendsaway from the blue LED chip to provide a consistent white point alongthe phosphor area. If the phosphor is infused in the top substrate, thetop substrate may be molded or otherwise shaped to have varyingthicknesses for controlling the effective phosphor thickness.Alternatively, optics may be formed beneath the phosphor to provide moreuniform illumination of the phosphor by the LED chip.

For improved heat extraction, any portion of the bottom substrate (whichwill be the highest surface when the light sheet is attached to/in aceiling) may be metal.

Any portion of the light sheet may be used as a printed circuit boardfor mounting a surface mount package or discrete components, such asdriver components. This avoids the use of costly connectors between thepackage/component terminals and the conductors in the light sheet.

FIGS. 54A and 54B illustrate one way to encapsulate the LED dies afterbeing laminated between the top and bottom substrates. Any of theembodiments may be used as an example, and the embodiment of FIG. 20B isused to illustrate the technique.

FIG. 54A is a top down view of a portion of a transparent top substrate740 with holes 742 for filling spaces around the LED dies 744 (shown indashed outline) with an encapsulant and holes 746 for allowing air toescape the spaces. The holes may be formed by laser ablation, molding,stamping, or other method. Representative conductors 748 are also shownformed on the top substrate 740.

FIG. 54B is a cross-sectional view of a laminated light sheet 750showing a liquid encapsulant 752, such as silicone, being injected intothe empty space 753 around each LED die 744 through the holes 742 in thetop substrate 740. The injector 756 may be a syringe or other toolnozzle typically used in the prior art to dispense silicone over LEDdies before mounting a lens over the LED dies. Air 758 is shown escapingfrom the holes 746. The syringe will typically be a programmedmechanism. By using the encapsulation technique of FIG. 54B, thelamination process is simplified since there is less concern about theinsulating encapsulant preventing good contact between the LEDelectrodes and the substrate electrodes. Further, the viscosity of theencapsulant may be low so that the liquid encapsulant fills all voids inthe space around the LED dies. Any excess encapsulant will exit from theair holes 746. When cured, the encapsulant will seal up the holes 742and 746. Curing may be by cooling, heating, chemical reaction, or UVexposure.

The encapsulant may include phosphor power or any other type ofwavelength conversion material, such as quantum dots.

As an alternative to using an injector 756, the liquid encapsulant 752may be deposited using a pressured printing process or other means.

FIGS. 55A and 55B illustrate another encapsulation technique used toensure that the space around the LED dies is completely filled with anencapsulant. The embodiment of FIG. 20B will again be used in theexample, although the technique can be used with any of the embodiments.

FIG. 55A is a cross-sectional view showing a blob of a softenedencapsulant material 760 deposited over the LED dies 762 prior to thetop substrate 764 being laminated over the bottom substrate 766. Thereis a small reservoir 768 formed in the bottom substrate 766 forreceiving excess encapsulant to avoid excessive internal pressure duringthe lamination process.

FIG. 55B illustrates the softened encapsulant material 760 being pressedand spread out within the space around the LED dies 762, with any excessmaterial overflowing into the reservoir 768. The space around the LEDdies 762 may be, for example, a rectangle or circle around the LED dies,or the space can be an elongated groove.

All the light sheets described above are easily controlled to beautomatically dimmed when there is ambient sunlight so that the overallenergy consumption is greatly reduced. Other energy saving techniquesmay also be used.

The light sheet of any embodiment may be used for overhead illuminationto substitute for fluorescent fixtures or any other lighting fixture.Small light strips may be used under cabinets. Long light strips may beused as accent lighting around the edges of ceilings. The light sheetsmay be bent to resemble lamp shades. Many other uses are envisioned.

The standard office luminaire is a 2×4 foot ceiling troffer, containingtwo 32 watt, T8 fluorescent lamps, where each lamp outputs about 3000lumens. The color temperature range is about 3000-5000 K. If low powerLEDs are used (e.g., model SL-V-B15AK LEDs driven at 20 mA), asubstitute luminaire would need about 580-620 chips for equivalence tothe DOE CALiPER benchmark troffer. Assuming chip prices in the range of3-5 cents, the total chip cost would be $17.50-$31. If the chips areoperated at higher currents, say 30 mA, then the total chip count couldbe reduced by approximately one-third. Power conversion/driverefficiency is about 85%. Overall then, the lightsheet efficacy (120 V ACto total lumens out) would be 78-86 lm/W at 20 mA drive current and 3.2V (compared to the benchmark T8 troffer performance of 63 lm/W).Accordingly, the invention can provide a practical, cost-effective solidstate substitute for a conventional 2×4 foot troffer, while achievingimproved performance and enabling a wide range of dimming. The inventionhas applications to other geometric arrangements of light fixtures.

The various features of all embodiments may be combined in anycombination.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skill in the art that changesand modifications may be made without departing from this invention inits broader aspects and, therefore, the appended claims are to encompasswithin their scope all changes and modifications that fall within thetrue spirit and scope of the invention.

1. A bidirectional lighting device comprising: a first plurality ofnon-packaged light emitting dies having electrodes; at least a firstsubstrate and a second substrate sandwiching the light emitting dies andforming a light emitting structure having a first light emitting surfaceemitting light from the lighting device in a first direction and anopposing second light emitting surface emitting light from the lightingdevice in a second direction different from the first direction; andconductors formed on the at least first substrate and second substrateelectrically connected to the electrodes of the light emitting dieswithout wires for connecting the light emitting dies to a source ofpower.
 2. The device of claim 1 wherein, the first substrate has firstconnection locations electrically connected to first conductors formedon the first substrate, wherein each die has at least a first dieelectrode and a second die electrode, the first die electrode beingformed on a primary light output surface of the die, wherein some of thedies have their first die electrode aligned with and electricallyconnected to an associated one of the first connection locations on thefirst substrate without wire bonds, wherein the second substrate hassecond connection locations electrically connected to second conductorsformed on the second substrate, wherein other ones of the dies havetheir first die electrode aligned with and electrically connected to anassociated one of the second connection locations on the secondsubstrate without wire bonds, and wherein the first substrate and thesecond substrate have light output surfaces for emitting light indifferent directions from at least the primary light output surfaces ofthe dies.
 3. The device of claim 2 further comprising an intermediatelayer between the first substrate and the second substrate.
 4. Thedevice of claim 2 wherein the first substrate and the second substratedirectly contact each other with no intermediate layer between them. 5.The device of claim 2 wherein at least some of the dies are connected inseries by the first conductors and the second conductors.
 6. The deviceof claim 5 wherein the at least some of the dies are connected in seriesby the first conductors and the second conductors interconnecting thefirst die electrodes to the second die electrodes.
 7. The device ofclaim 1 further comprising: an intermediate layer over the firstsubstrate, the intermediate layer having holes corresponding tolocations of the dies on the first substrate such that the dies aresurrounded by walls of an associated hole, wherein the plurality of diesare sandwiched between the first substrate and the second substrate,with the intermediate layer there between, wherein portions of the firstconductors and portions of the second conductors connect at least someof the dies in series without using wire bonds.
 8. The device of claim 1further comprising at least third substrate sandwiched between the firstsubstrate and the second substrate, the third substrate having areflective layer for reflecting light out through the first substrateand the second substrate.
 9. The device of claim 8 wherein the thirdsubstrate has third conductors formed on its surface for interconnectingat least some of the dies in series.
 10. The device of claim 1 furthercomprising a reflective layer between the first substrate and the secondsubstrate, wherein some of the dies are located between the reflectivelayer and a light output surface of the first substrate, and other onesof the dies are located between the reflective layer and a light outputsurface of the second substrate.
 11. The device of claim 1 wherein thedevice is formed as a light sheet.
 12. The device of claim 11 whereinthe device is suspended from a ceiling so that the first light emittingsurface faces the ceiling and the second light emitting surface facesaway from the ceiling.
 13. The device of claim 1 wherein the device is aflexible light sheet having transparent opposing light emittingsurfaces.
 14. The device of claim 1 wherein the dies are vertical lightemitting diodes (LEDs).
 15. The device of claim 1 further comprising awavelength conversion material provided on or in the device forconverting light emitted from the dies to white light.
 16. The device ofclaim 1 wherein at least some of the dies are connected in series by theconductors internal to the device within outer boundaries of the firstsubstrate and the second substrate.
 17. The device of claim 1 whereinthe device is part of a 2×4 foot ceiling fixture.
 18. The device ofclaim 1 wherein the first substrate and the second substrate each havedimensions of approximately 2×4 feet.